There are people on both sides of the spectrum (short distance road track AND all mountain-100-miler trail ultras guys) who seem to have no idea what they are talking about. There is a scary lack of perspective.
I'm not saying i know what I'm talking about, but I've seen both sides from moving up on the roads/track into MUT Running. Disclaimer: I haven't done 100 miles...yet.
To give Mamu some credit, he won the 2012 World Mountain Running Champs (12km, granted it's not like the IAAF cross country champs) in 1:01:34. I was back in 1:05:55, Joe Gray was back in 1:06:20 and Rob Krar was back in 1:08:03). But that was only one race and i'm only comparing a couple runners that people might know. He's a class runner in the MUT scene and I have no idea what his track/road times are, but I'd guess he'd really kick my butt at any distance from the mile to any ultra.
Kilian, on the other hand has also always kicked my butt..but it's always been at an ultra distances in fairly "extreme" environments. UROC, was probably the most tame course we did, but it was still 66 miles going up to 12,000 and powerhiking in a foot of snow. And Krar and Dakota and Clayton beat him that day. At The Rut 50km, I powerhiked a mile up one of the "off-trail" ridges and split like a 23min mile (all-out). Kilian powerhiked about 3 minutes faster that mile and caught me. He passed me on an uphill when I was breathing very hard (he was not breathing very hard) and it went exponential from there. But then again, my Vo2max isn't 90 and i suck at powerhiking. On technical downhills he is waaaayy better than almost anyone. I suck at any sort of downhill running...even on smooth roads at -3% grade.
I think what you'll continue to see is "the rise of the MUT Runner" where faster road/track guys will move into these mountain-ultra-trail races. Course records will fall, FKTs will be reset, and guys like Kilian will get a run for their money. It won't be a flood of guys though, because anyone who is looking to tap a sub 2:10-2:12 road marathon can still make more money (or make the Olympics depending on what country you're in) at the shorter distances. There's an injury risk in MUT Running, and there is also a learning curve... I have no doubt in my mind that some of the guys who smoke me at shorter distances on the road/track would also smoke me at some ultras. But I'm only a 2:16:52 guy and I just started training in the mountains/trails 2 years ago.
I've said this before and I'll say this again: it all comes down to developing specific musculature for efficient running economy on variable terrain. Being fast on the flat track at 5km doesn't always translate so well even for a race like Mt. WA (which is only 7.5 miles on a road) but all uphill at an average 12% grade. That's why you have a guy like Zach Miller (31-min 10km runner in college) beating a guy like Ryan Bak (13:40ish 5km I think?) in that race (Sorry Ryan, just wanted to use an example). I would also point out that that same Zach Miller is the only guy who is undefeated against Rob Krar in an ultra. Some of this ability is what you're born with and some of it is how you've developed your musculature through specific training. We could look at metabolism has well as a deciding factor...
So there is a lot of variation in the ability of runners (from Vo2max to 10km/5km and marathon PR times) and how they will actually do in a MUT Running event...(Which could be anything from a Vertical K to a 100-miler on the track or in the mountains like Hardrock). Then again, with events like this I don't think it really matters what the actual distance is...a 50km in the mountains could take just as long as a 50-mile race like JFK. So course profile (elevation, terrain) determines your ability to perform more than anything else. That variation between courses and MUT Running distances will keep any one athlete from dominating everything and it will also lead to fields where not all the top MUT Runners line up against each other.