Modular Smart Accounts will be helpful for smaller things like adding signature schemes, but it would still be a burden to users to have to make sure their wallet has all appropriate modules installed (and the correct version of the modules) before every action. Consider the scenario where a user wants to sign two intents that require different versions of the same module. The user would potentially have to go back and forth uninstalling and installing different modules before each intent, and you couldn’t overlap intents that have incompatible requirements.
I would also say that there is slightly better security in electing code to run once via ERC-7521 rather than having it become a permanent part of your wallet. In ERC-7521, if a bug is identified later down the road, only wallets that have unsolved intent floating in the mempool would be effected.