At a 4% withdrawal rate, Let's Encrypt would need $100 million invested to not need to ask for funds in the future (assuming they don't drastically increase their operating expenses).
Governance and oversight is mandatory though; Wikipedia has net assets almost near $113 million [1] and requires less than 600 servers to operate (plus colo costs, connectivity, technical staff, etc). On the other end of the spectrum, OpenStreetMap costs $118k a year to operate [2].
[1] https://en.wikipedia.org/wiki/Wikimedia_Foundation
[2] https://twitter.com/floledermann/status/1057254329290235907