1. The well-known resilience of mail can be largely attributed to its use of MX records to specify the use of backup servers and/or load balancing. SRV records are essentially a generalization of MX records for any service, not just mail, so using SRV records would bring this benefit to any protocol which uses them.
2. The problem of “www” or no “www” would be entirely eliminated; no need for non-standard so-called ANAME or ALIAS “records”. Just imagine – if, in an alternate universe, mail servers hadn’t used MX records, it would then have been necessary to have an A record on the bare domain name, and later, when the Web was created in this alternate universe, the web server would have to be the same server as the mail server, since both now would use the bare domain name. In our universe, this position is now solely occupied by the HTTP protocol, which causes lots of pain in the DNS when redundant (and harder to update) A records need to be put on the bare domain name just for HTTP. Not to mention IPv6, which, for HTTP to work, requires an additional AAAA record on the bare domain.
3. The “everything is on port 80, let’s reinvent everything on top of HTTP” problem could certainly have been entirely avoided, and could be slowly fixed, by using SRV records. (A server concerned about clients needing to traverse old-style restrictive firewalls could provide a low-priority port 80 server as a fallback.)
① A DNS RR for specifying the location of services (DNS SRV) https://tools.ietf.org/html/rfc2782