Once a FOSS ecosystem exists, it can always be maintained as such.

What may theoretically happen is that the same client won't be able to join both an open-source controller and a controller from Tailscale-turned-evil.

I suppose that corporations, especially those without huge IT departments, will and do happily pay Tailscale the reasonable money, and have their secure VPN just working. For them, it's no different than paying for Zoom or Office365, only cheaper. They totally do not want to depend on in-house networking expertise.

So I think Tailscale will be doing well financially,

They could, but let's cross that bridge when we get to it. You're trying to justify the dislike of a product because of what they might do. That's a pretty weak argument.