This will sound corny, you can practice mentally in your head against a opponent, in your dreams. Everything comes from the mind, create your self a fighter's mind. I think it's possible to learn standup without sparring, just my theory lol.
About 4 years ago (when I wasn't actively training), I would have agreed with you. Now, I would say that it is just a theory based off of philosophy and not actually training.
To the TS: You cannot teach yourself real stand-up skills for fighting. If you are looking to learn how to punch, yeah you might find a modicum of success in learning how to throw something you think
looks like a good punch with a heavy bag. But the whole caveat that you have to deal with is that your opponent (no matter how untrained they are) are not going to stand still, or fall into your rhythm. Visualization may be a great way to wrap your head around technical concepts, but it does nothing for the actual application of those concepts.
Find a gym and a trainer. Learn from them and work hard at what they teach you.
It doesn't matter if you agree or disagree
... As long as I don't bore you and I spark a moment of thought, my goal is achieved
Queng leon queng tigre ecu tacacut, queca pa? - Pampangan Mandarigma Motto