I don't think I would buy another e-bike that was obviously an e-bike. There are too many questions about where they are "allowed" to be ridden, and too many busybodies out there. That isn't to say that a bike that one of us would detect as being an e-bike would be out, just the obvious ones with huge obvious motors and protruding batteries.
There is very little ability to use a given battery from one bike on another bike, with few exceptions within a bike brand or perhaps with an occasional less common motor. I think it is unlikely that this product will evolve so that batteries become interchangeable; there are so many examples of common electronic items where that has NOT become the case, such as cellphones, digital cameras, etc. It's pretty obvious with the standard battery sizes out there for consumer goods (AAA, AA, 9V, C, D), that new consumer goods have not moved in the direction of battery interchangeability, rather they have tended to have internal rechargeable lithium ion batteries that are not user accessible. Rechargeable batteries for consumer goods have been sold as a feature, but in many cases what they are is a part of an overall strategy of product obsolescence.
Integrated batteries in e-bikes should be designed in a way that they can be removed and reinserted into the bike by the end-user; many or most are. When these batteries reach their end of life, a lot of the bicycles will be considered dated and/or obsolete, and owners will choose not to buy another battery but rather will dispose of the bike. That may be unfortunate and it may contribute to landfill waste, but the obvious fact is that e-bikes are going to evolve over time, and probably become progressively cheaper. My guess is that what will happen is that they will become much lighter over time and given the choice of spending $500 for a new battery to continue using a 60 lb, 5-year old bike, that people will choose to spend $1000 or $1500 to replace the whole bike with a new one weighing substantially less.
Finally, to the extent that people do want to resuscitate their older bikes with dead batteries, the currently-existing few shops that will custom rebuild your bike battery, will increase in number and this service will be the way that people keep these older bikes on the road long after the manufacturers have ditched them (or gone out of business) and the original batteries can no longer be found. You will ship your dead battery to one of these places and they will refurbish it with new cells and send it back to you.