I'm not sure how a bike can be wrong assuming the sizing is right. No two bikes will ever feel the same, are some people trying to get a bike hat feels like their last? (What's the point in that?), and there is always an element of getting used to different bikes and just adapting. The two MTB's I've got are completely different, completely different geometries and intended purposes, riding positions, weights, handling characteristics, and I'll choose the bike best suited to the ride I'm about to do. I guess I'm just easy!
I think you only need to demo if you haven't got a clue what kind of bike you want, to decide what category of bike you actually need.
Demoing may help if you have a short bike history list, where what's down on paper can't be related to real life, and you dont have a capable sales person to make clear what is down on paper says out on the trail.
I really dont think you need to demo to get the correct geometry/size. If you're trying out something new and different, it's always going to feel different and not like home.
I believe everyone needs to live with a bike to actually find out what it's like, buying, testing, learning, moving on, buying, testing learning etc is just part of bikes to me. Plus your riding style changes and grows, what you want/need is constantly shifting.
Not blowing my own trumpet, i get to try out a lot of bikes, i haven't ridden anything for years that hasn't ridden exactly how i thought it would. I've learnt that by taking the plunge and living with bikes.
You also get those serial demoers that never find anything, usually because they're looking for the moon on a stick. I think the sooner that you grasp, no matter what bike it is, designed for whatever purpose, there's things it's going to be good and bad at, there isn't a bike that exists that has zero bad traits, it's just impossible.