I know a guy like that, but his reason is basically that MoM didn't give him anything compelling, and his highest level character is still only 43. Basically, he had no reason to buy MoM. I'll be surprised if he doesn't buy SoM, though, simply because he wants to do the skirmishes (which start well below his current level) and because it's dirt cheap at $20.
I'm a little curious about this "massive amount of level 50 content" in SoA, because I must have missed it. Yes, there's certainly more at 50 than at 40, but that's expected in a level-capped game. By the time Moria shipped, most players were getting pretty bored at the cap - when I did /who counts on Elendilmir, I often found that I couldn't get an accurate count for level 50 characters because Hunters and Champions had more than 100 level 50 characters apiece! Capped characters made up a large chunk of all characters playing, and it's not a bad assumption that a lot of the others were alts. That's why there was so much screaming for more high-level content, and excitement for an expansion that would raise the level cap and provide 10 new level's worth of content.
A year later, it seems like the number of people who don't want to progress past 50 would be even smaller. I'm not saying they don't exist, but I'd be very surprised if the percentage of the player population was very large. In any case, those people you're talking about still wouldn't skew the results of a LOTRO survey against the "Turn XP Off" feature. Sounds like they'd probably be supporters

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Khafar