If there was some big central fund to pay for CI for many open source projects, it would probably be for-profit companies donating much of the money to support that. Is that much different to for-profit companies making in-kind donations of free build time?

You might hope you'd get a broader pool of companies paying in to the fund, so you're less dependent on any one company. But unless the donors throw almost infinite money at you, you would soon need to make some kind of restrictions on which projects can benefit and how much they can use per month.

The best I can think of to do is to make your CI config a relatively simple wrapper around generic tools, so it's easy to move when you need to. This is often a trade-off with things like parallelism and caching, but at least if you know it's a trade-off you can make sensible decisions.