I mean Reddit Gold was a cool idea back in its heyday.

But yeah, funding it via donations comes with some implications in regards to cutting non-donation costs.

Say you have a single donation tier, on patreon or so. If push comes to shove and you get too few donations, the following would be perfectly understandable things to happen:

• To cut costs, non-donating users are rate-limited for their interactions.
• To cut staff cost, defederation is vigorously employed to both cut external content to review and also marginally cut server cost from the actual federation updates.
• To further cut staff cost, non-donating users will automatically be temp-banned if reported with no prior investigation, and placed in a low-priority review queue.

These all sound grotesque, but also understandable if you think about it, in particular the last one as personell cost is always really significant. Yeah you could argue "might as well go paid-only", sure. But to a point free users are okay, they just have to accept that in any "maybe" situation, they'll always lose out due to costing too much vs paying too little (nothing that is). "You get what you pay for" would be a way of putting it.