1) I do wonder if Blizzard never got rid of talent trees how big they'd be now. Would they be akin to the trees of Path of Exile? :P
2) if Blizzard kept giving us new spells every expansion the amount of utility we'd have as well as the amount of balancing Blizzard would have had to deal with.
For these reasons I see why Blizzard gives us borrowed power, but I wouldn't mind seeing some of the batshit possibilities from over 8 expansions if they never did. :P
I think its good to find a balance, borrowed power is fine as long as we get to walk away with some of that power when we leave rather than be wiped like a clean slate by the time the expansion ends.
I mean seeing all that effort after the Legion expansion just go away before BfA was very sad, I kind of wish that tree our Artefacts had remained in some way and attached itself to our character.