If you want to use two wheels to explore mountainous, off road terrain. Then you generally travel more and thus get to see MUCH more using an e-bike, and the experience is generally a fair bit more pleasurable!...
Will I eventually phase out the conventional bike ...?! Has this happened to anybody on here?? I'd like to do the road biking as a pure workout to increase fitness but use the KTM to go for blasts off road which have the emphasis on fun and enjoyment, not some gruelling workout like mountain biking used to be....!!!
However I've found that while you can increase your fitness using an ebike, in general, you push yourself more, and thus get fitter on a conventional bike. I suspect that if you're (very) fit to start with, fitness will decrease if you use the ebike exclusively.
(although if you're unfit to start with, you may NEED the ebike to allow you to get out there, and that's the most important thing)
So it depends on what you want to get out of cycling.
Personally, I think having both types of bikes is the best of both worlds, then you can pick whichever one suits your purpose for that particular day. (and often I'll do something like do a hard workout or commute on a conventional bike one day, but then take the ebike the next to allow myself to do an extended recovery ride).