This is a rather weird point to make, considering the way Wikipedia has handled the collection and usage of funds. Investing in its technical infrastructure would be a less controversial usage of their money than what they are doing currently.
As a theoretical exercise, I'm not sure that's strictly true. If you're reliant on donations, then income is never guaranteed. If you suddenly fall short of targets, maybe you have to lay some staff off. But stopping your license fees means you then have to pay someone to switch to an alternative.
Avoiding recurring payments you can't easily stop makes reasonable financial sense to me. Particularly for a business that doesn't sell anything.
As a theoretical exercise, you could also choose to "lavish" funds on the developers of the OSS platform instead of a recurring license fee for use, accomplishing a more outreach oriented use of funds than paying non-profit "employees" north of market with remarkable packages. If funds get tight, you can simply stop laving on the OSS, while enjoying ongoing use. Such a license can definitely be arranged -- I've done it at scale.
Let’s say you need a feature in nginx. You can either pay for an nginx plus license, or fork the oss nginx and develop the feature in-house.
Many people assume that developing it in-house is a one-time cost. However, in my experience, code is not an asset but a liability for which you will have to pay interest in the form of maintenance. More specifically, in this case there will most likely need to be adjustments made to the in-house extension as the oss nginx version it was built on top is being updated over the years.
My point here is that you should probably treat in-house code as a recurring cost, because in practice it usually is.
The question then becomes which recurring cost is higher: The license or the in-house maintenance?
