If only it were as easy as that.
Open sourcing it might be the most straight forward option, but there could be complicated intellectual property issues behind the scenes that make that prohibitively difficult, or expensive.
A paid option? Isn't that what every single commercial business attempts to do? The option of using a product or service in exchange for paying money. The failure rate for businesses is huge. What makes you think a paid option would actually work? It isn't a sure thing, nor are donate buttons, and in the time you need to take to test pricing models, tweak copy, increase conversion etc, you continue to lose money each month, and have no guarantee of success.
I think you drastically underestimate the effort involved in "post a paypay link or a bitcoin link".
What about the cost of compliance? The cost of accounting? Customer service? Cost of mitigating risk? There are a heap of costs to running a business. How do you know if the donations you (probably won't) get will cover them? Then what? You've lost more money.
At some point you just have to call it a day.