> Installing an app that silently intercepts SMS/MMS data is a persistent technical compromise. Once the app is there, the attacker has ongoing access.

The motivating example as described involves "giving the scammer everything they need to drain the account". Once they've drained the account, they don't need ongoing access.

Persistence allows the scammer free license to attempt password recoveries for every account the victim could possibly have. Other banks, retirement accounts, the victim's email account.

When the victim's relatives send them money because they need to eat and pay rent after handing everything over to the scammer, the persistent backdoor lets that money be drained as well... You're underestimating the persistence and ruthlessness of the scammers.