At least you have the benefit of being able to ask these questions now, before any possible dementia could set in.
There may or may not be a connection between your condition and dementia. But dementia is a popular topic of study and tons of information has already been gathered about it. You can use this information to protect yourself from ever getting it.
For example, PubMed has lots of studies on the effect of exercise on risk of dementia. It actually reduces the risk a lot, especially if it is vigorous exercise.
Even if you are genetically predisposed to getting dementia, you can stave it off a long time. My grandfather was a health nut. He eventually got Alzheimer's, but that was when he was 95. He'd lived on his own until he was 90. So he had the genetic predisposition to get it, but his healthy habits completely suppressed it until the last year or so of his life. It wasn't his cause of death, either, so the worst he got was where he would vividly remember stuff from when he was a boy in the Great Depression. Remembering what he ate for lunch? Not so much. Still, not too shabby, I'd say.
You aren't doomed, no matter what anyone says. You're smart enough to understand those studies you found linking schizophrenia to dementia, so you have the capacity to understand other research you find that might tell you how to reduce your risk. The answers are out there. Everyone is going to tell you that nothing can be done...but when you go look at the studies for yourself, you will have a very different opinion.