The list of sites in the beta phase has now reached two pages in length, which probably serves to "cloak" some of the newer beta sites, and some of these sites have been going on the better part of two years. If they were going to make some substantial traffic move, they would have done so by now. Right now, it feels like there's essentially simply a "Tier 2" group of sites that are established, but low traffic enough that Stack Exchange can't be bothered to move them to the main site. Which, as I understand it, isn't the purpose of either Area 51, or the notion of beta testing sites.
I looked at the metrics reported for the 53 beta sites that are currently more than 10 days old (the metrics for brand new sites are bonkers), and looked at some of the relationships between age of the site and metrics for "success". Here's what I found:
Questions/Day: This is, in my mind, the currently most flawed metric being used by Area 51. None of the beta sites reach the "healthy" threshold, and to be perfectly frank, if the 15 questions/day threshold is going to be continued, sites actually trying to reach non-beta status (rather than languishing in at-best a perpetual beta) should pack up, go home, or seek other alternatives. But I stray from the point.
If we're expecting older sites to "age into" higher questions/day, we're going to be waiting a long time. A simple linear fit of the two (more complex fits didn't perform better) shows a significant, but tiny increase in that ratio. An increase of 0.0042 per day older. To put that in perspective, that's an increase of 1.53 per year. At that rate it will take the most successful site (by that metric, it's Code Review) another 1047 days to reach a target of 15. That's another 2.9 years in beta.
Answered Percentage: There's really no relationship at all, and most sites are doing fairly well on this metric anyway. It's not what's holding sites back.
Avid Users: Unsurprisingly, this one does have a very strong association with site age - an "avid user" is added, according again to a simple linear model, every 2 days or so. Given reputation takes time to accumulate, this is as expected. But it also can't be what's holding sites back - a wealth of the older sites (and some newer ones, like Christianity) have 200+ avid users, and there's an even greater number nearly there.
Total Users: Again, unsurprisingly, older sites have more users. As there's no entry for what's considered a "good" number of total users, I can't evaluate what's considered a "successful" site, but there's several older sites in the tens of thousands of total users.
Ratio of Questions to Answers: There's another very small linear relationship - another waiting years for a single digit increase in the ratio. Beyond that, officially 2.5 is "good". Only 7 of the 17 sites more than a year old have a lower ratio than that, and none of the oldest 5 do. This, again, can't be what's holding them up.
Visits: Visits is another very strong relationship - about 4 visitors are added per day, according to the fit. But, once again, 1,500 is supposedly "good". There's 7 year old sites that are above that threshold - some by a considerable margin.
What's the point of all this musing? Letting betas run clearly has the intended effect - the older sites are, by and large, stronger. But some of these sites are now quite old for betas, have established communities, and it appears are waiting for the "Questions/Day ratio" - or some metric that's hidden behind the curtain - to get across the starting line.
My assertion is that the 15/day number is a flawed one - eventually niche sites simply won't be able to reach it, and it either needs to get adjusted, or sites that don't get there need to be put out of their misery. But is there really any reason to keep say, Personal Finance, Startups or RPGs as betas? They've got large numbers of avid and total users, a healthy Q&A climate, 1500+ visitors per day...what purpose is served by keeping them off the main site? What's the path offered for them to reach main site status given they haven't already, and the one metric where they're struggling is something that, based on the evidence, they won't reach in a meaningful amount of time?