I was just going through the same RFC searches, you saved me the trouble of posting them all :)

And POP has been around since 1984 (!) and POP3 since like 1991, both as usable standards. I'm not saying IMAP doesn't have its advantages, I'm saying that things take forever to change, and POP3 is ubiquitous. IMAP's advantages are advantages for the end user, not the people who implement it like ISP's...