You can still do a lot after the fireworks show. On a recent visit, we found a 40 minute posted wait on Peter Pan. However, there were only about 10 guests in line and the wait was less than a minute,
Next up was 7DMT. Posted wait was 90 minutes with an actual wait of 10.
We finished the night with 4 rides on Space Mtn in 35 minutes.
We have been staying until park close for years. In short, the experience varies widely.
While crowds generally diminish, waits are not always this short right after the fireworks end.
In our experience, there is typically a surge of folks on the rides just after the fireworks end. This is especially true for rides that are close to the hub, like Buzz. One moment the queue is empty, then people stream in, but sometimes the posted wait still says 10minutes. Even BTMRR can have a significant surge.
If the weather is nice, the parks tend to stay more crowded. People tend to leave when it is very hot, cold, windy, wet. People tend to stay later on nice weather, holiday weeks, and during special events.
I still have old paper FP with like 2am return windows stamped on them. We generally DID ride the attractions that late, we just opted to keep the paper FP as proof. Usually come 2am the standby wait wasn't very long.
I hope WDW will go back to offering more (non-upcharge) late hours!