Maybe 75 is too few for the most dedicated, but that is not the target of achievements.
Actually, that is the exact point of achievements. They are supposed to give you something to work toward. With over 600 pets in-game as it stands right now, 75 for a leveling achievement is far too low of a number, especially after they added the Celestial Tournament which requires you to have a set amount of level 25 pets before you can even queue for the content. With even more new pets coming in, they could have at least added in one more achievement for leveling pets.
They are supposed to be something that appears reasonable and doable for your average person starting at zero.
Not all achievements appear reasonable and doable for the average person. How many achievements are there for completing heroic raids (and their or high-ranked arena teams? Hundreds and those certainly aren't reasonable or doable for the average person. I don't see why pet battles should be any different. They might be considered a leisurely activity for most, but for some, they are very much a big part of what we do in WoW. To me, a raid or battleground is a leisurely activity and I don't see why we should be punished for choosing something else to do with our time.
Not all achievement chains need to get added to cumulatively to extreme, unreasonable numbers. Just becomes meaningless and insurmountable for most people when that happens.
You're right. Achievement chains do not need to get added to unreasonable and extreme numbers....but they do. Take the Legendary Pet Battler achievement. It requires you to win 5000 pet battles. For a lot of people, they are never going to accomplish that. Yet, it's still in there. Same goes with the Legendary Pet Brawler achievement that requires you to win 5000 pvp pet battles. That an achievement that I (who loves pet battles and spends most of his time on WoW doing pet battles) will never achieve. I simply do not care much for pet battle pvp, so I will never earn that achievement. It's still there, though, for the people that will work towards getting it.
I, personally, do not believe that adding in a few achievements for leveling 150 pets to 25 and/or 300 pets to 25 is really going to break anything. I would just like some recognition for the time that I put into leveling those pets.