The believe that highly technical consensus changes (even if they're not large) don't have an exploit, you'd have to have confidence in a LOT of public scrutiny IN PRACTICE. Just being open source is not enough Remember the heartbleed bug that was introduced in 2012 but not widely recognized as an exploit until 2014.

I don't know if Monero is making the right tradeoffs. You have to consider that the old protocol may not be that great either, indeed it may even have deliberate exploits (I don't know of any, apart from all of the public limited ring size issues) and the situation would improve by replacing it.