Yes. Nothing wrong with borrowed power as long as it's actually interesting and ties into your spec (so not soulbinds/conduits), and is kept within reason instead of having 5 separate layers of it. Using a talent row gives them very clear constraints on how far they can go with it, which is a good thing.
Sure, I'd be fine with that as well. Just have a Legion artifact to fill out each expansion (with a weekly cap per spec and artifact knowledge) and use that for borrowed power. I do think there's value in talent trees that you can use to adapt to different situations, but most specs have had such poor talent balance for years that it doesn't really deliver on that. That way they can also build each spec's kit in a way where they're all balanced compared to each other, rather than giving some specs everything baseline while others have to choose one or the other in their talent tree.
The devs want the most optimal way to play the game to also be fun. (Note: I'm not saying nor implying they always succeed.) Micromanaging your conduits, or more likely running an addon to micromanage your conduits, doesn't add anything to the game. It's just extra fiddly stuff. I completely agree with that design goal and decision.
The same thing applies to swapping covenants. This is a MMO. Many players will go to great lengths and endure tremendous annoyance for even a small advantage. Best to limit that stuff so nobody's tempted. Same reason why they removed world buffs and consumable stacking. You used to be able to spend hours farming consumables and world buffs before a raid for a real advantage. Since it was possible, and really helped, many guilds made it mandatory. But that SUCKED, and the devs removed that stuff, and they were right to do it.
It's Blizzard's game, they get to determine what they view is a meaningful choice within their game. They're not saying that it's meaningful for you, but it is to the game designers. That's how game development works, you have your vision and try to communicate it to your audience. Whether or not they succeed is up to your interpretation. Clearly in your case they failed. Whereas in my case, they succeeded. I find many of the choices made meaningful. Sure potency conduits are not the best choice, but the paths and other two conduits are meaningful to me.
“You can never get a cup of tea large enough or a book long enough to suit me.”
– C.S. Lewis
It would be interesting to see what covenants all the casuals who quit picked but the sites that "track" covenant choices conveniently leaves out all data older than a month. Probably because it doesn't fit their narrative.
Also how many people stay on after the story is "Finished."? Let's face it many people were playing wow for the follow up to the WC3 storyline involving Arthas, Legion, Illidan etc. Which culminated with the battle on the Frozen throne and the death of Arthas. If wow was a single-player series or a movie series or a TV series. The fall of Arthas would have been the endpoint as the story was wrapped up.
Legion was defeated in BC with Kil'Jaeden getting the Sargeras treatment in the Sunwell (As far as anyone knew at the time Sargeras blew up in the Well of Eternity destruction). Illidan was defeated in the black temple. The last part the scourge was finally defeated with Arthas fall at the hands of Tirion and the adventurers. For many it was the right stop off point from the story standpoint.