When I was a child I used to love concept cars, I loved how they were much better than regular cars.
I would listen carefully to when they would be available in the market, most of the time that information wasn't mentioned, sometimes the date was so far in the future that it wasn't interesting, by that time we would already have flying cars!
With time I started noticing that none of the concept cars from my childhood were on the streets, not even close. Worst was when a really bad version of them was available for sale, the worst deception ever.
Now when I see a company showing off with a concept car I think the opposite, that company is running out of real ideas or has lost the ability to execute novel designs, and tries to justify it by showing shiny things that it knows will never see the light of day.
Why I'm talking about concept cars? well, because there are different types of concept cars and different types of software prototypes, but they are almost the same.
The prototypes that require you to accept things that are never going to be feasible but are sold as if they are possible are the worst. Either because they violate basic laws of physics, materials, safety, regulations, performance, user experience or just because they focus on a single concept by disregarding others that are required when the thing migrates from prototype to production.
If you are upfront and tell that the prototype is an exploration of "what if we push this dimension to the extreme", I'm ok with it, it's a learning experience, you may learn a lot about that dimension and how it relate to others, what are its limitations and so on.
But the prototype should be clearly marked as such.
The other useful prototypes are as learning exercises, I like to build throw away prototypes as my first approach to something, as a way to learn more about something and have a better idea for next time. You should also mark them clearly as such.
The third useful prototype is the initial stage of something you want to grow into a product but want to show the potential along the way, you may throw some versions away in the early stages because you learned something that required a big reformulation and it's easier to start from scratch than to refactor it.
But from the beginning the prototype should be grounded in reality, what's possible and how the main concept relates to other features that may be a year or more in the future but they are going to be required for the prototype to turn into a product.
You can't have a slow/complex prototype in the early stages unless you have a clear idea on how it's going to get faster/simpler, small optimizations here and there aren't going to cut it once the performance/complexity penalty of all the extra features starts creeping up. You have to think up front how those other features will fit once you reach that point.
Of course, if you are building something new, at some point you will be in a new territory and some things may require some reformulation, that's good, but at that point you need to have buffers of performance and other metrics and a simple core architecture and code to be able to solve those problems, even if not in the optimal way. That's why you built this new thing, to push the boundary a little further.
If you get the chance to build a new prototype after the current one turned into a finished product, you can then incorporate those new insights at the beginning to be able to push a little further than before. Rinse and repeat.
What you should not do is to build a prototype that starts to fall apart even when only solving the single problem you care about and call the remaining just an implementation detail or "left as an exercise for the engineers".
It may sound like a contradiction, but to build good prototypes you have to be good at building complete products, otherwise you can't guide your initial design on constraints that you never experienced. A prototype should eventually be the foundation of something much bigger and complex, you can't build that on unstable foundations. If not you, somebody on the team must provide the experience that comes from completing, polishing and maintaining something that survives the contact with reality.
As Mike Tyson said: "Everybody has a plan until they get punched in the mouth."
Or put another way:
A complex system that works is invariably found to have evolved from a simple system that worked. A complex system designed from scratch never works and cannot be patched up to make it work. You have to start over, beginning with a working simple system.
There's only one dimension you can ignore if you have the time/money to do so, and that's price: see the Experience curve effect. Just don't be too early ;)
inb4 appeal to visionaries, check Vannevar Bush's memex, it was a full design, it was easy to see how you could build it, even when some technologies were in the future, everything was feasible. Ivan Sutherland Sketchpad Demo was a complete runnable thing, same with Douglas Engelbart's Mother of all Demos, Xerox PARC's Alto and Bret Victor's Dynamicland.
Don't fool others, but most important, don't fool yourself.