I suspect this has arisen out of evening EMHs. The powers that be have decided that they do a "good enough" job keeping non-Resort guests out of EMH without clearing the parks that they can do this for Parties too.
The problem is that for EMH, there's almost nothing open that isn't staffed by an individual CM (the attraction entrance CM or character handler), so they *can* generally check most of the time. For a Party, there's enough stuff (parade, fireworks, forecourt show, ...) that will be impossible to police.
If MNSSHP is a disaster, I can imagine park hours revised again for
MVMCP to reinstate the changeover hour. But, from what I hear, the MK Ops folks are convinced that they don't need to manually clear the park, and their EMH experience has convinced them of it.
And, really, I don't think it will be a disaster. There will be a few hangers-on, just like there are a few suspiciously old 2 year olds, very suspiciously old 9 year olds, and people who disregard the note in the dining brouchure about not using child credits for adults.
But, the majority of Guests seem to abide by Disney's written policies, even though there is no enforcement mechanism. And, really, when Disney can cajole people into following the rules rather than force them to, that's what they prefer, because no CM wants to see an unhappy Guest, even if that Guest is clearly in the wrong.
Many people take advantage of this inclination---spend 20 minutes in guest services of any park, and you'll see a fairly constant parade of people trying (often successfully) to wheedle park-wide FP re-entry passes for every little real and perceived slight. But, it makes the day more pleasant for the vast majority of guests who are reasonable, and Disney has decided that that's the preferable route.