I've been around a while - 30+ years of being local(ish) to and visiting WDW regularly. So I've seen no FP, OG FP, FP+, and pandemic-no-FP in action. I can attest that with no FP, StandBy waits are shorter and the lines move nearly continuously (with exceptions to shows, where a huge number moves in and out all at once, of course.) But by "shorter" I do not mean 10 minutes (though that absolutely did happen on slow days). Back in the Olden Days, a busy day meant deciding whether to wait an hour or more to ride Space Mountain later in the day, get there at opening to have a shorter wait, or skip it. FP/FP+ spoiled us, and I get that it would be really hard for some folks to wait a long time for something they never had to wait long for before. It's an adjustment, for sure.
About % of capacity, you really have to remember that it was very, very rare for parks to ever hit full capacity. And that full capacity is ENORMOUS. MK alone is near 100,000 at full cap. An average not-super-busy-but-not-totally-dead day at MK in non-pandemic times is somewhere in the neighborhood of 40-50% of total capacity. So if that's where cap is now - say 45%ish - yes, it feels "normal" on a sold-out day... because it is!
I have a lot of faith in WDW's ability to plan for increasing capacity. They've known for a while that more resorts are reopening, so they've had plenty of time to think about how much more needs to be open in parks, and how much more efficiency needs to increase to handle higher capacity. More eateries open = spreading more people around. Increased throughput on rides (reduced distancing in line, seating all seats, loading every vehicle, etc.) = more people moving through an attraction in the same amount of time. These are things they've tinkered with for many, many years to maximize efficiency and reduce costs on a daily basis: they know how it works!
The one big kink in the chain, maybe, is staffing: can they increase staffing adequately to manage more people in resorts and parks? 80% of CMS returning who were asked back doesn't strike me as super great when it's so hard to hire new people right now. That's a big "remains to be seen" for me.