i think that its the hot air that doe the damage. Perhaps, the dry hot hair melts, or better yet, fuses the tips of the fibers in MF. This is way causes them to get stiff/ hard. In boiling water, however, there is not burning, rather its steaming the towel to release any caked on chemicals. In the process, it is expanding the fibers, which is why I noticed my MFs to be fuller and softer. Everything is relative though.
If you have any doubts, maybe try it on one of your towels and see if you like the results. If you boil for about 5 min, I promise your towels will be fully rejuvented.
FYI- Most of my prior stains are still in the towels, so this method didn't help in that respect.