If that is the standard, political parties will just start claiming that all their opinions are factual (as happens in practice, coincidentally).

If we had an algorithm to sort fact from fiction then that would be fine, but we don't. Our method for sorting fact and fiction is to ask an expert. Experts are not reliable in political situations because there are experts who are willing to believe anything for money.

Particularly for economic questions where it is likely that the experts have a financial stake in one political outcome.

It's not only money. People are ready to believe (or to claim they believe) in things they think will make the world a better place (for some definition of better). This where the real partisanship lies. Tell a white lie, turn a blind eye, jump to conclusion, ... are all easy things to do if they justify moving the needle towards a world you truly think it's better.

People of course have wildly different standards w.r.t what "better" means.

And once shaped, the goal is hard to change. It takes more than facts or logical arguments to change one's idea if what the ideal world looks like. It's part of your identity, it's intertwined with your community.

I didn’t mention opinions which are claimed to be factual matters. I mentioned factual matters. The fact that people can lie isn’t exactly new.