At what point do "free forever" tools need to start charging, and how do you decide?

Running something that's currently 100% free with no ads or paywall, and I keep circling the question of whether that's sustainable or just deferred pain. Server/hosting costs are low right now since a lot of the processing happens client-side, but bandwidth and time investment aren't free even if compute is.

I've seen founders go three different directions: stay free and monetize through something adjacent (donations, sponsorship), add a paid tier for power users only, or eventually paywall the whole thing once there's an established user base. Each seems to have a different trust cost with existing users.

What actually triggered the "okay, now we charge" decision for people who've been through this?