Third-party SMS and MMS messaging apps already exist on Android, such as the free and open source QKSMS[1] and Simple SMS Messenger.[2] Signal also used to support SMS and MMS on Android until last year.[3] There isn't a shred of evidence that these non-default SMS/MMS apps increase spam to any measurable extent compared to SMS/MMS apps that are preinstalled on Android phones.

[1] QKSMS: https://github.com/moezbhatti/qksms

[2] Simple SMS Messenger: https://github.com/SimpleMobileTools/Simple-SMS-Messenger

[3] https://www.hackerneue.com/item?id=33179047