I have sympathy for non-profits. Did you ever use Gmail or Google Drive? Same concept.

If they're smart enough, they'll look for greener pastures, if they double down that's when my sympathy ends.

Non profits likely can't afford a team of tech workers to maintain opensource infrastructure. Avg tech worker salary in the states is $100k, you'd need two or three staff for coverage/to avoid key-person risks, so you're looking at $200-300k to retain internal teams to use opensource tech, in salaries alone. Opensource doesn't mean 'free' too, as many businesses will generally require some sort of support/enterprise agreement for risk management/audit reasons -- so they'll pay lic fees on top of the salaries.

The technology supports the business, but it isn't the business -- sorta like a pizza shop may need a brochure-website to advertise their services/take orders, but their actual business is pizza. Someone who's specialising in pizzas, shouldn't need to learn how to maintain an opensource server stack. If you get a 'golden goose' staff member, who's some university student genius that loves pizza and will happily maintain opensource infrastructure on the side, then your business is totally fucked if that person leaves because finding a replacement with similar skills is nearly impossible / very expensive.

They can outsource it to a third party, as most businesses do. And then that third party goes through the same general calcs, and settles on big box tech solutions for similar reasons.

I don't think its as clear cut as you seem to imply.