Fringe approaches would not be considered "fringe" if they were proven to have an effect. Even if it improves your survival rates by merely 1%, it would be still used as part of modern medicine if there was nothing else.
The problem is that the quack makes you believe that it will work, so you're still wasting time trying the "treatment" and potentially suffering its side-effects (not to mention the financial impact). It's worse than not doing anything, since at least the latter means you've accepted your fate and can enjoy whatever time you have peacefully instead of being busy with a quack treatment.
Furthermore, modern medicine isn't inherently hostile to "alternative medicine". If you think about it, all the potential treatments being researched in labs right now (including for covid-19) are still at the "alternative" stage, and if they end up being proven to work they simply become "medicine". What modern medicine is hostile to is unproven, or proven not to work treatments.
If you think you actually have an "alternative" theory that isn't quack, you are welcome to do your research on it to at least rule out any existing reasons why it couldn't work (using existing medical literature), and if the theory still stands by then you are free to engage with the mainstream medical community or study, become a researcher and then test your theory in a safe and controlled environment so the outcome is actually valuable (and will influence further research even if this particular theory doesn't work) and not just anecdotal evidence.