@zkzkz:
-Ryanair has high load factors. I am not doubting that. Ryanair has also landed in so much regulatory hot water on and off that it's a wonder no country has come up with a way to throw them out.
-Also, the answer to that question is "how rare are we talking?" If there's reliable off-season availability (say, in Jan/Feb and Sept) that corresponds to lower load factors (and/or availability on lower-travel days of the week), then people will. If availability is limited to extreme odd-and-end legs and you can't find a round trip to save your life, not so much. The question is one of relative value.
--Corollary to the above: A lot of airlines retain availability, just not at the lowest award bucket, which is going to retain some value in and of itself. Generally, I'm seeing a trend towards buckets of "Saver", "Standard", and "Full" or something along those lines. So if there's regular availability, just at a higher cost, that's less of an issue for both sides (travelers can still get utility from their miles, albeit not as much, and the airlines can get more miles off their books without doing as much damage to customer relations).

As to award prices not being pegged to ticket prices, that is changing increasingly (though the link is not perfect and there are sometimes bonuses for buying a full-fare ticket as well). Virgin America and, I believe, Southwest are mostly or entirely based on cash spent (Southwest includes significant bonuses for not buying a low-end ticket) plus tier bonuses. VIA does much the same (Amtrak hasn't learned this trick yet, but they also only just figured out the value of letting customers choose a higher fare class on the NEC so I expect them to get there in due course), and I believe that Delta has switched over. American is likely to take their time (due to the merger).

Moreover, via their credit cards a number of airlines are behaving in a mixed manner. An Upper Class ticket on Virgin Atlantic is likely to get as many points off of the credit card as you'd get from the "mileage" side of things (if not more of them, which I think is the case on the higher buckets). From what I can tell, airlines are moving in this direction in stages since tying earning to miles traveled simply doesn't make much business sense.

@Kagehitokiri:
Nice link. I think that analysis is a bit too simplified (travel-earned miles may be a liability in some respect, but if an airline sets things up to whereby they're getting other benefits from the program through "Program Miles" then those miles have a reason to still exist; if nothing else, those miles do allow an increased level of data aggregation on their part in terms not only of customer behavior on the airline...but through partner offers as well, making what they have useful in terms of monetizing data).