"Why" we collect hasn't changed all that much, although some people's goal may be to collect to battle rather than collect for fun. Either way, we still want to "catch them all". The most significant change that I've noticed is "HOW" we collect.
In the past, collecting was mainly a "do it yourself" activity. You could enlist the help of a few friends, but in general you had to do the grunt work. From farming mobs, questing, or grinding reputation - there was a very clear path to the pet you wanted. There weren't many workarounds or options to collect some pets, and if you wanted it you had to jump through the necessary hoops. At the same time, that was part of pet collecting's allure. A few (and I stress the few) pets that were difficult to find, but so satisfying to collect.
These days, post-Pet Battles release, not only has the number of pets skyrocketed, so has the interest and desire to collect. On top of that, most pets are now cageable which makes it possible to trade/sell them. Combine these changes together and what you end up with is a situation where it's somewhat unreasonable to expect a player to adopt the mentality of "do it yourself" due to the exceptionally large number of pets to obtain. With cageable pets, buying and selling has become a much more viable option for collecting, and it helps alleviate the strain of facing RNG and the grind of over 500 pets.
Some collectors have taken to grinding gold instead of the pets themselves so that they can simply purchase what they seek off the Auction House. For them, it's a more effective and efficient use of time and energy. Which is understandable. There are only so many hours in a day and days in a week, but so so so many pets. Sometimes RNG can be unkind and risking the roll each time may only mean time lost. So it's only natural that players seek out an "easier" and more direct way of doing things.
Don't get me wrong, though. I think it's great that collectors now have another option when it comes to acquiring pets. You could do it yourself or pay someone else to do it for you (not literally hire someone, but purchase a pet from them). The market is open for everyone and that's perfect for spreading pets around and reaching a wider audience. Even those who may not even enjoy pets all that much can still participate by buying low and selling high.
But now that utilizing the Auction House is a popular and pretty efficient way of collecting pets, it means that old-school collectors will need to adapt to this shift in HOW collecting is done. Farm a pet? No, farm the gold to buy the pet. This is where I run into an issue.
As someone that adores these in-game critters, I'm ultimately in it for the pets. I don't want to farm for gold; it's not fun and it's not what I'm after. It's just a means to an end. Call me old-school, but I really do enjoy the hunt and search for a pet. I love the journey of it all. Coming to the end of that adventure only to find a pet is extremely rewarding and satisfying. Well, at least it was.
However, I do have to admit it HAS become pretty tiring. Every patch has been explosion of companions, and trying to research and collect them all by yourself is a lot more time consuming, tedious, and frustrating than in the past. The sheer number to find with each new release kind of dulls the experience, and that gratifying feeling of coming to the end of a hunt is instead replaced with relief that I can finally move onto collecting the next pet on my list (or doing something else).
So while I don't want to make gold to just buy everything, it seems as though the situation is slowly pushing me towards that direction. It just doesn't make sense to mash my face against the wall to get to the other side when I could simply walk around it.
I remember spending a few days for about 6 hours straight each day grinding for the Firefly pet. Could I do that now though? For 20-30 pets a patch?
Perhaps my age is starting to show/kick in, but I'm finding time to be very precious now. If there were only a few to collect every other release, it would be similar to pre-MoP collecting. There would be more time between each one, and it would feel as if there was more time to search before resorting to buying it. (Of course that would also mean less content for collectors, which is a huge bummer.)
With how things are now, it's completely unreasonable to spend hours for each one out of 10, 20, 30, etc., and would probably result in me going crazy in the process. Maybe I had more time in the past, or maybe I was simply younger, but I just can't seem to put forth the same amount of energy into "doing it myself" these days.
Yet if I can't seem to grind out the hours and battle the RNG like I used to, it leaves me with only one other option: buy the pets. The problem for me is that I'm terrible at making gold. Saving gold, I'm great at that. But making it? I just can't do it. My interest isn't high enough in farming for it, even if I do realize that it will lead me to what I'm actually interested in (pets).
The shift in "how" we collect has really put me in a tough spot, and with this evolution of collecting, I may just get left behind. If I can't find it in me to sit through the grind for 20 pets a patch or farm up thousands (maybe hundreds of thousands) of gold each new content release, I'm afraid my pet collecting days will be over.
I think that buying pets will steadily become more and more popular than trying to farm your own, and with each new batch of many many pets, it will only become a better option. I mean, if you have to collect 20 or even 10 pets, and must face RNG or a long grind for all of them, would you? What if the option to simply make enough gold to purchase them off the Auction House was there? I think most would take the latter path, even if it removes most of the journey and adventure of collecting. Spend time and energy to battle RNG (a risk/chance) or make gold and buy the pet (a guarantee)? Yea. I'm pretty confident that many will stick with the guarantee.
What would "solve" my issue? Perhaps if there were fewer new pets each patch. But if there are many new ones, maybe lower the RNG. Then again, I love seeing so many new additions, so I don't think I'd want Blizzard to cut back on how many are released each time. The RNG could be adjusted, but that's tougher to balance with everything else. They could make the pets uncageable which would force us to go out and hunt them ourselves. That would be unfair to many, though, and I'd rather see lots of people enjoy companions.
In the end, it's probably just me. I just need to adapt better and make the change to how I collect along with everyone else. It's too taxing to try and stick with the old-school mentality of "do it yourself", and it would be more reasonable to just buy low and sell high. Unfortunately, it's all easier said than done for me.
I don't plan on giving up pet collecting any time soon, but I do find that I'm struggling more than before. It's put a slight damper on the experience, but I enjoy my collection too much to just let it all go. I'll either make a change in myself or power through it. One day my dissatisfaction with myself or the situation might get the better of me, though, but all good things must come to an end eventually.