I’m very skeptical that mCaptcha would actually work besides perhaps temporarily slowing bots down due to being niche. How expensive can you make it without hurting legitimate users? And how expensive does it need to be to discourage bots? Especially when purposefully designed bots can actually do the kinda math we’re talking about in optimized software and hardware while legitimate users can’t.
Barriers are relative. Everything that makes it slightly harder will stop a large chunk of bots, since bots aren’t able to easily adapt like humans can. Plenty of very basic bots are in fact stopped by lack of emails.
But yeah, email verification is heavily more so that you can verify they have access to the email, and thus the email is safe to use for things like password resetting. Without it, webmasters can get swamped with complaints about people getting locked out of accounts or the likes because they signed up with the wrong email.
In theory, you can also go further by only allowing email providers that have anti bot mechanisms, but it’s difficult to maintain that and it will always exclude some legitimate users.