I don't use Tailscale because I don't trust their key distribution, and this open source project would solve that, but it might undermine Tailscale's sustainability.
This would be a shame because Tailscale is working well with the open source community: open source clients, working well with distros, working well with Linux DNS stack, supporting a more P2P secure Internet, and documenting their well through it.
Also, Tailscale offers OAuth through large corporate providers; I'm not sure Headscale is going to support that. (Actually, this is why I don't use Tailscale for my private network: I don't want to depend on an external OAuth provider.)
Companies looking into this will pay Tailscale.com service. You really need commercial support if you plan a large enterprise deployment. Tailscale even now offers a self-hosted version of their service - for those with concerns about using the public SaaS.
Both look like great products, I just don't need what they're offering enough to accept their downsides.
So many users will not have even considered the paid version anyway, and their participation will give tailscale a higher marketshare and thus more viability.