It's one thing to provide security in the kindergarden. It's another thing to provide security in the jail. So far Apple's track record is kindergarden security. They do have tech in place, but how that tech would resist big bad world outside the wall remains to be seen. My guess would be that there will be apps breaking the jails in the first years and 10 years later things will settle on and it'll become relatively safe to sideload untrusted apps.

Of course if one's smart enough to only download apps from reputable websites, then the only worry will be privacy issues which are probably not important for most people