MMS could've been (and still kindof is AFAIK) the only way to send rich text semi-anonymously for money with carrier billing ie not requiring platforms, app stores, and sign-up. Is was also blocked on Android due to the Stagefreight bug. MMS was used at lot for ringtones and wallpapers before the Smartphone era.
I am guessing they already knew GPRS/UMTS and data plans were the future, hence they invested in iMessage. MMS already had an expiration date. Quite sure they only added it because of the PR disaster it had become.