VCFirst off, the parks operate on a phased closure basis. I forget the exact categories but it's along the lines of:
Phase 1 - non resort guests with day tickets only cannot enter (parking may be suspended first) and will be redirected to another park
Phase 2 - all non-resort guests refused entry, no park hopping for onsite
Phase 3 - only those with fps, ADRs, other reservations allowed in
Phase 4 - full!
In 2016 there was only a brief phase 1 closure, where they redirected cars to Epcot but allowed people to travel by monorail and still enter MK. So not really a closure at all! I know on NYE there wee 2 hours of traffic jams trying to get into Disney property in the afternoon, but no closures. So if you have an adr etc you'll be fine. Also info was out everywhere when it looked like MK was heading for phase 1.
I too was worried planning, but in the event it wasn't a problem. Be prepared, have a touring plan. Even if you don't like optimising, like me, evaluate your plan to see how it looks. The crowds are big, it can be slow but we still had a great time..