Not an E-bike owner and not someone who pays attention to performance so dismiss as appropriate....
I have had problems with high exertion cycling such as steep hills
Is that because of the combination of pace and gradient or just the gradient?
or extended periods of cycling fast on slight gradients.
Where is the urge to cycle fast coming from? Do you ride with others and feel that you are slowing them down? Do you want to ride with others and avoid doing so?
Is it a case that you are not taking an ageing body into account and trying to do what you used to be able to do?
I'm of the opinion that riding a bike is a great thing to do and that anything that gets us riding more has to be a good thing.
However, there's a lot of people who think something like "If I just had X I'd do more of Y" and that isn't always the case. Especially on a focused forum like this where we're exposed to new and improved things all the time.
I am worried that I might get too used to the assistance and not ride my conventional bikes enough afterwards.
Can I ask what would be the problem if you did stop riding your conventional bikes? Is it just the fear of losing fitness?
Fitness, on its own is a funny thing. I think there are different kinds of fitness. Someone who races for an hour or two has a different fitness to someone who pootles all day long. There's mental fitness and physical fitness, too.
I'd imagine, and some of the responses above suggest, that physical fitness can be maintained or even improved by an Ebike. As with most things, it will come down to how you use it.
I'd think that if you focus on physical fitness, and physical fitness only, that you'll be pointed in one particular direction. If you take a wider perspective where enjoyment, relaxation, exploration, sociability, de-stressing or just the simple reduction of mental barriers and all the other things a bike can bring you, you may find that there are a number of options.
I met a group last summer on a collection of gravel/mtbs all out for a ride in glorious country, the "oldies" on E-bikes, the "youngsters" on acoustics and everyone was enjoying themselves. I know that because a lot of them slowed down to chat with me. If anyone was to drop out of that group because they felt that they couldn't keep up and that an Ebike would hamper their overall fitness I think that would be a great shame. Sure, they might be "fitter" by riding on their own but fitter for what?