At that point why not just rent the whole bike and not need to bring the large object onto the plane?

Because ones bike is a carefully selected item perfectly fitted to ones body shape and preferences, whereas the battery is simply a line replaceable unit.

That's such an edge case though.... what about my fine tuned piano? Or my aquarium? At some point personal luxuries don't have to be accommodated for

Even as an avid cyclist who fully understands geometry and the personal connection one may have to one's bike, I agree with you.

Privileged enough to fly somewhere specifically to ride bicycles? C'mon, I think you can find a good enough bike to rent when you get there.

Absolutely you can. The places in France and Spain I've flown to just suggest you bring your own pedals, so they match your shoe cleats; they'll fit them before your hire. You can usually bring your own saddle too. It's far more convenient than bringing the bike.

I've also done it the other way, my main bike has S&S coupling so I could bring it aboard the Eurostar. For touring I prefer my own bike, because I have the racks set up for my panniers, but when I do that, I prefer travelling by ferry/train.

I love it! Even for the lift/shuttling-bike-park crowd, I bet many places would happily install cleat-matching pedals (or your own), your saddle... maybe your grips if you are picky?

I think the intersection between people who ride e-bikes and cyclists who obsess about bike fit is fairly small.

You totally have the option to fly with a regular muscular powered bike.