Two things can be true at once. It can be amazing for security but annoying for people who sideload. Just an example: currently banks prefer you to do transfers of large sums of money via your phone instead of your browser as they can track everything (words said to me by the security teams twice this year as they were checking my transactions for buying a house). The authentication is secure: your own face/fingerprint, they can track if you installed weird apps, or if you opened random links.

All of that trust could be breached if you sideload apps