I agree with others that this seems like an attempt to pivot to “music as a higher art”, e.g. the rumored Two Set Academy, like a Tonebase competitor. Which is an absolute shame, because I think they did wonders to challenge that status quo and make classical music fun.
[1] Shameless, but if anyone is curious about an adult hobbyist learning classical guitar, I’m livestreaming almost daily: https://youtube.com/jcpractices
A secondary benefit for me is that Purelymail is cheap ($10/yr). But a major downside is that the CalDAV implementation doesn't yet support Accepted/Rejected scheduling responses. It's fantastic that they have CalDAV at all, but this one feature would seal the deal for me.
I don't mention it to suggest you could directly compete on that unlimited-users value prop, but it's an example along the lines that max_ suggests.
One that comes to mind is integration with recent ActivityPub / fediverse stuff. There might be cool design space there