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.
Avoiding recurring payments you can't easily stop makes reasonable financial sense to me. Particularly for a business that doesn't sell anything.
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?