As a bonus, they could make the app curation system extendable, so other groups could run vetting programs. By default your device would only have Apple as a trusted vetter, but as part of a management profile or something you could also trust apps vetter by some security research group, or Epic, or Valve, or whoever. That way security isn't all-or-nothing.
And of course, if anything slips through and your layman user OKs all the warnings and installs a malicious app, it's still operating in a sandbox, and can't do much more harm than clicking a shady link and ending up on a scam website can.