PL does not prevent or cure RF interference, it just adds selective earplugs. Nor does it allow several users to share the same frequency channel at the same time.
If two RF signals are on the same frequency at the same time, there will still be interference. If the signals are close to the same amplitude, the user will hear a heterodyne or beat note. (Google “radio capture effect” for more information on how that works.)
The only real way to share a RF channel is by properly spacing the two repeaters or stations apart. This is why we engage in voluntary frequency coordination. (i.e WAR). From a repeater user-end perspective, using the least power necessary to carry out the desired communication is paramount to enable effective frequency re-use. This is why that is in the rule book.
If everyone ran 100 watts all the time, you’d likely be clobbering the input to several repeaters in the state, even though you might not be bringing them since they likely all have different tones.
This is why when the band opens up, it can be tricky to get into your own local repeater with a HT. The repeaters receiver might be being swamped by strong signals from other hams in other adjacent areas.
Keep in mind the repeater can hear things you can’t as it’s up very high on a tower.
Now going back to the notion simultaneous frequency use. It is actually possible. But you have to do this digitally. Once the voice is digitized you can time-slot the channel. Google “Time division multiple access (TDMA)” for more info. Or see this video: http://hak5.org/episodes/episode-816
The newer Mototrbo radios are TDMA.