Unfortunately, if the carriers impose the same attachment limit, it really won't help. The limit on MMS is largely artificial (considering the phone and the MMSC talk HTTP...sending an MMS is literally an HTTP POST).
RCS is its own technology. If everyone in a chat uses RCS, SMS and MMS are not used at all. It is like iMessage - but based on an open standard
The reason there is an attachment limit is because the carriers enforce it in the first place. It isn't a limitation of the MMS standard.
As such, what would stop carriers from enforcing the exact same limitations on RCS?