This is effectively what Reddit Gold is. In a perfect world, Reddit would be completely funded through Gold, therefore eliminating the pressure on higher ups to increase revenue through ads or other less than ideal revenue streams.
Unfortunately, Gold has not been enough to fund the site (if the sidebar on the front page is to be believed) for a long time.
reddit gold is voluntary tho, I was wondering what if you force it like when you enter reddit it make you choose between spending x$ or accept ad, I wonder if people will still use it?
They would not.
Why would people do either of those things when ad-blockers exist?