Not completely true. If the Basic Auth password is stolen and that password is only for an email account, then it can only be used to access email just like a OAuth2 token. What you are saying is only true if the same password you use for your email is used elsewhere, which is a big no no. You could say users won't do that, then you have the option to force application passwords which will allow you to make forced unique passwords to use used on a per app basis.
Limited lifetime? Most have issues with long-lived access tokens. If you had short-lived tokens, then one would need to login again all the time to renew the token. You could use silent authentication, but that bypasses any check to make sure you are the proper owner of the token by using the MFA login account. I have never been asked once to renew my GMail or Office 365 OAuth2 token for my email account when using a IMAP client connecting to those services using a OAuth2 connection method.
Lastly, yes they support IMAP and SMTP, but are forcing the use of OAuth for IMAP/POP which breaks decades worth of software and scripts unless they are updated to support OAuth2. Additionally you have to use that services token management to manage the tokens (like revoking or setting how tokens expire) and access rights for those tokens. This adds another layer of complexity. The question is how long will they support IMAP and POP before they depreciate those also for being so called "insecure" or "legacy".
These are common reasons they say OAuth2 is secure and Basic Auth is not:
1) OAuth2 is encrypted so it is more secure: Basic Auth can be encrypted and forced using SSL/TLS.
2) OAuth2 uses multiple unique tokens so it is easily to remove access to a specific application: Basic Auth can use forced application passwords which could be used to create a auto-generated unique password for different applications. Basic Auth would require more manual intervention and is not forced.
3) OAuth2 supports MFA: Basic Auth can support the use of 2FA.
4) Tokens can be set to have limited lifetimes: You can enforce Basic Auth passwords to have limited lifetimes and be rotated.
The advantages which basic auth for email accounts does not provide a solution for:
1) You can assign limited permission rights to a token, where basic auth is a all or nothing access.
2) Passwords are stored server side only when using OAuth2, while basic auth stores the password in the client and server-side.
What OAuth2/OpenID does do is enforce best practices of using encryption, MFA, and per app unique identifier while basic auth these must be enabled to be forced.