I think there is some misunderstanding here. IMAP (Internet Message Access Protocol) and POP3 (Post Office Protocol) as there name suggests are messaging protocols. Thunderbird, Outlook and a hundred others similar programs are email clients. Google, Yahoo, your own ISP and many other, provide access to internet services including mail.
Should Google drop IMAP, it wouldn't stop Thunderbird and all the other email clients from receiving IMAP messages from those providers of Internet mail who still supported IMAP. Conversely should Thunderbird drop IMAP there are a hundred other email clients who would likely be grateful for you to install their software.
I personally cannot imagine Google or any other of the big players dropping IMAP because while POP3 is likely the more popular protocol for home use and frequently set up by defauly, IMAP is very much the preferred protocol for business use and I'm not sure such providers would actively encoarage their users to start look elsewhere. Peter