There are a lot of smaller providers that will eschew some features, require a fairly limited integration surface (was the case with a lot of things with Slack integration before Teams' COVID explosion, for instance) to keep their support and development costs down
The best estimates I could find have TS at a 1-2% market share. And that was my point: You suggested "well if Tailscale can get away with not supporting SAML, anyone can!" and I think that's wrong. 98% of the market already is buying Tailscale's competitors, they can hardly do worse
I'm sure they can juice that a good deal more, but eventually they'll have to start picking off features that they have been avoiding to this point. Will that be SAML? Maybe not, most companies are on Entra ID which supports OIDC. The real tell will be when Tailscale either goes public or sells to private equity. When their backs are to the wall and they need to squeeze out another percent or two in growth and they have a big customer that must have SAML, then they'll do it