You could say that about discussion forums too. But reddit and HN would disagree. Quality has to be managed and maintained. It's plausible to me that by designing the right mechanisms you could scale AL and maintain quality.
HN is very industry specific. It would be like saying HN is disrupting forums. It's not. It has a very specific audience. Reddit has lots of above average and also has below average forums too. So, yes, the AL jobs board could become the board for all jobs, but then, it'd hardly be better than job boards now.
Seen another way, let's say the top talent is 10,000 individuals. There are way more positions and candidates than that out there. So if they became the Reddit of job boards, they'd have tens (dom)/hundreds (int'l) of millions of applicants in search of tens (dom)/hundreds (int'l) of millions of jobs. Neither the applicants nor jobs are going to become "great" just because they are on the Angellist board.
Yelp is an example of a system where it'd have been easy to be cynical about their system scaling up. It's nowhere near being a perfect system, but I use it frequently and I'm able to repeatedly find great restaurants there. It does work reasonably well at scale for me.
And the whole site is based on people actually spending tens of minutes crafting reviews. Who'd have thought it'd work out as well as it does?
It's somewhat hard to argue with a generalization like this. It's like saying: you give loans to 100 people meeting some very specific criteria, and the return rate is great. When you scale it to a million, the return rate isn't the same any more.
Of course the return rate won't be the same with 1 million people. But if you are grameen bank (http://www.grameen-info.org/), you institute mechanisms so keep the return rates high. That doesn't mean that there are no defaulters at grameen bank. It just means they were able to work creatively on a problem and actually meet their metrics (maybe sligtly relaxed) at very high scales.
It's non-trivial, but not impossible.
I think a measure of low quality is: do users/participants actually stop using it because of bad quality? It's a point of debate, but for me HN's quality is as good (probably better) that it was several years ago. Just my opinion.
Seen another way, let's say the top talent is 10,000 individuals. There are way more positions and candidates than that out there. So if they became the Reddit of job boards, they'd have tens (dom)/hundreds (int'l) of millions of applicants in search of tens (dom)/hundreds (int'l) of millions of jobs. Neither the applicants nor jobs are going to become "great" just because they are on the Angellist board.