DCA has only hit capacity twice.
Last year,
Disneyland hit capacity on Dec 26, and 27th and the 31st, and then Jan 4th.
If the fire marshal closes the parks, there is no admittance even for dining reservations. That said, with the exception of New Years Eve, when the park does hit capacity, and the fire marshal does a hard close, as soon as enough people exit the park, they will allow people to start reentering. You can tell if they are expecting a possible hard close, there will be green taped lines in the middle of the esplanade. They will rope off the turnstiles and trickle guests in. Generally, even if there is a hard close, it will reopen after the afternoon parade.
Over Christmas 2 weeks, AP holders have never made up the majority of guests. When there was the premium pass, during that 2 weeks period, APs represented about 18% of the guests, and last year when they reduced the APs to only Signature Plus, they still represented 12%. What people don't realize is it is a huge travel time for people without APs. The park is a completely different vibe. I am very used to doing the park at this time. We've gone the 2 weeks of Christmas for over 20 years. This is going to be our first year we won't be there (well, we will be there Dec 22-26, then heading home so we can head to WDW from the 29th to the 9th, then back to DL the 11-15....)
It will be crazy busy, you will have Rose Bowl groups, you will have groups from all over the world attending, you will have families coming who will think no one else comes to Disneyland at Christmas because "everyone stays home with their family". I've had people say "It won't be busy because all the APs are blocked out." APs are blocked out because they expect it to be over the top busy. I've had people sit down on CHristmas with me (no tables and I invited them to sit to eat) and they couldn't believe how busy it was, they assumed it would be empty.
Dec 23-25 will be busy. Dec 26-Jan 1 will be crazy busy. Just based on past experience.