MMS video and picture sizes are size constrained by the carrier in the profile. As many carriers already allow 4MB or even unlimited sizes it is purely an issue of carriers being cheap and lazy and leaving old limits on MMS. It would probably even be profitable for carriers as MMS already counts against data and RCS allows the larger sizes anyway. Yet Google prefers to roll yet another new protocol and complain instead of getting the carriers to just fix MMS configuration issues to solve the biggest ongoing complaints.
Just as a data point from outside the US: a 4MB MMS is about 5$ in Germany, even on a 30+$/mo base price package. Sending that to a group call multiplies the cost with the number of recipients.
MMS (and in all likelihood RCS) is a complete non-starter for this market.
