Users demand it. You can't have a popular VPN app without Apple support (because at least one person in the org will have an iDevice), so you have to do it. I made another comment in this thread about my experience building EteSync.
That's one of the more annoying parts about Apple being the gatekeeper to 40% of the US population (and in effect, to 100% of businesses). As a developer, you are just stuck with no way out.
Many apps (e.g. EteSync and WireGuard) are almost useless if they don't work for everyone within a certain group. A more extreme example is a messaging app. Will not having iOS support for a messaging app lose you 40% of your users? No, it will lose you 100%.
In WireGuard's case it's maybe less obvious than messaging, but if WireGuard doesn't work on macOS, it's enough to have one Apple user in your whole organisation in order to make it a non-viable solution.
> So if bigcorp wants OS X WireGuard support, they should be able to pay handsomely for it.
Who says they're not? A lot of the companies on https://www.wireguard.com/donations/ ship their own macOS software. Just because the Wireguard Mac app is free doesn't mean nobody's giving them money that's earmarked for Apple development.
Video editors, designers, and sound mixer are a few example professions where users mostly use Apple products. Most companies have designers.
Additionally, companies don't choose their whole software stack based on their VPN solution. They would just change a VPN solution if it's incompatible with what's there.
By now, video editors and sound mixers are heavy windows users, because there's no halfway endurable Apple machine that you can purchase that supports 128GB of RAM and 8+ CPU cores and NVIDIA CUDA. Because like it or not, almost all video editing plugins use CUDA for acceleration.
You'll lose more than that. If there wasn't a viable Apple solution, half my dev team wouldn't be able to use it, so 100% of my org wouldn't be able to use it because I can't maintain half a solution. You'd be left only with tinkerers. I'd say you'd have lost about 95%.
Does the free and open source Wireguard need to be a popular VPN app? One benefit of being more popular is that they get more contributions, but given that they barely get enough contributions to fix macOS-specific bugs as it is, it's not clear that the benefit outweighs the costs.
Apple and Apple users respond to tangible consequences; appeasement doesn't seem to be working, and it doesn't seem to be benefiting the project either. Like OP said, it's magnanimous of the developer to do this but I don't think "users demand it" is a great justification, nor is it quite in the spirit of open source.
Why build it in the first place if you don't want people to use it? Also, a lot of VPN services are moving to WireGuard, they will hopefully contribute to WireGuard development in the future. You can't really do cost/benefit assessment based on current contribution values. If you did that, no startup will ever start, and no open source project will ever be created, as upon creation the usage is almost always zero.
Windows & Linux users will still be able to use it. Most popular VPN services seem to develop their own custom desktop clients (they do this for OpenVPN); they will definitely contribute to Wireguard, but I'm not sure that they will contribute much to the desktop-specific parts of the "official" apps.
Edit: I should add that there is another cost/benefit assessment here: if Wireguard developers continue to appease Apple, Apple will continue to make life difficult for them as there will be no pressure for it to behave better.
You can't just look at the clients, you have to consider a VPN protocol holistically. Do you think that most VPN installations are going to run two different gateways side-by-side so that they can provide Protocol X for Platforms A and B, and Protocol Y for Platforms C and D? (Even worse: X for just Platform A, and Y for literally every other user.)
That's one of the more annoying parts about Apple being the gatekeeper to 40% of the US population (and in effect, to 100% of businesses). As a developer, you are just stuck with no way out.