I'm aware they wrap OSS, but they made it very, very easy to adopt and maintain for a large chunk of potential users. This requires significant effort and should not be undervalued, in my opinion.

exactly, which means setting up a vps, generating certificates, setting up some type of monitoring to make sure the tunnel is working, etc. I agree that wireguard is the best option, if you have the time and knowledge, but for some dev people that just wants to put up a webpage with a few users, tailscale/cloudflare is a much easier system to maintain (especially as it handles ssl for you as well - to some degree...).