@ Brattle Loop, I'm not sure what I'm supposed to be conceding here as I've repeatedly said that this project is a win-win. Boston area needs workers, South Coast needs access to better paying jobs. It's not about equity or owing anybody anything. It's that the largest population in Eastern Mass currently not served by commuter rail is the South Coast and there's some opportunities there to solve a few of those problems.
That it's the largest population center in Eastern Mass not served by the CR can be true and irrelevant at the same time. If the service is too poor, because of the limitations inherent in using the Old Colony routing (especially with no plan whatsoever for widening the single-track main), that the population doesn't really use it much, then it doesn't much matter how big the population center is, it'd still be under-utilized. (And may well kill Phase II, and with it any hopes of a proper-capacity SCR schedule and proper Buzzards/Cape service.) More to the point, the size of the population center is absolutely irrelevant when we're talking about cost-per-rider being as high as it's projected here. (At least some of which is probably down to the less-than-robust peak schedule depressing projected ridership.) "Boston needs workers" plus "South Coast needs access to better-paying jobs" (statements so generic as to be effectively unfalsifiable) does not add up to "we must build a rail connection at whatever the cost". Because the cost is considerable, not just in financial terms (though the cost-per-rider based on the state's own projections is
very high), but also in terms of what it does (hopefully temporarily, potentially permanently) to other (if smaller) population centers' prospects for service, as well as for the South Coast's accessibility itself.
The ridership projections themselves directly undermines the "Boston needs workers" prong of the argument (and puts a big dent in the "South Coast needs access" one). I'll freely concede that accessibility issues here pose a problem for assessment given that there's probably a decent cohort who live in the South Coast but don't seek employment in Boston because of accessibility, and some who are employed in Boston who don't live on the South Coast for the same reason, but unless you can point to something to indicate they'll shoot ridership through the roof (and, uh, the state can't) you can't count on them as a factor for projecting ridership.
What we're left with is a flawed (potentially permanently flawed) Phase I that still comes with a very-high cost per rider. That number is critically important, because it screams inefficient use of state funds even before you take into account the fact that Phase I also disrupts another potential service expansion that Phase II would not interfere with. The cost inefficiency is why the size of the population center is irrelevant; if other population centers can be more efficiently served (i.e. lower cost per rider) then, economically, those projects are a.) more efficient use of state transportation dollars and b.) implicitly harmed to some degree by money being used (inefficiently) for SCR.
You don't solve the "Boston needs workers" prong or get any kind of a win-win on anything by inefficiently using the transportation budget; if other projects can deliver better cost-per-rider numbers, they're by definition more economically efficient. South Coast is sort-of a winner, in that FR/NB get rail service (though with a fairly high risk of being stuck with permanently-crappier service than under Phase II), Buzzards/Cape are losers, and Boston and the Commonwealth (and its taxpayers) are actually losers in that they get X number of commuters at a very high cost-per-rider where other projects could potentially deliver that same number of workers more cheaply. Which means that the argument for Phase I in particular is not an economic argument, it's a political one about who deserves what. (Phase II I can get onboard with long-term growth and accessibility of the Gateway Cities being sufficient economic justification to the state.)
I'll make clear, I don't have a problem with FR/NB arguing that they deserve transit access. I have an issue with you criticizing discussion of the "deserves transit" line while essentially making that same argument in the same breath. Even Phase II is not immune; there are probably other transit projects with better cost-per-ridership (i.e. proxy for economic efficiency) than Phase II, meaning that it's ultimately a political "deserves" argument for that too (though that fact is largely true of all transit projects to some degree because we don't have unlimited money), but you continue to treat Phase I as if it inherently passes the cost-benefit test while simultaneously criticizing the "deserving" argument that is (in my opinion) the only reason it even has a
chance of passing the cost-benefit test. So in long-winded answer to your question, what you're supposed to be conceding is that the benefits fall primarily on the South Coast and the costs fall elsewhere, and to such a degree that criticism of the project is entirely justifiable on those grounds.
Some are advocating for Cape access and that's cool but absent some ridership projections I'm not sure how a much smaller population than the Taunton/FR/NB triangle would send more potential commuters into Boston not to mention whatever is going on with the Corps operating the bridge over the Canal. Regardless this project is nearing completion while many others haven't gotten off the drawing board yet so I don't think SCR is holding anything else up.
Obviously the lack of ridership projections makes life difficult when it comes to comparing projects. I should say I don't necessarily think it's likely that Buzzards/Cape would have higher ridership than FR/NB (because the population differential cuts against that), just that it's possible for that phenomenon to happen (and in this case, it might well happen seasonally given the Cape's heavy tourist skew). Cape service proper has to solve the Corps' who-knows-what with the canal bridge (someone, possibly F-Line, speculated they might just want to sell the dang thing and are annoying the state to get them to pay for it), Buzzards Bay service doesn't touch the bridge and does not depend on cutting through any Corps shenanigans. As for SCR holding things up, it's unambiguously holding up even consideration of Buzzards Bay service, because there's not enough spare capacity on the Old Colony main with SCR coming to provide an acceptable schedule to Buzzards. They were justifiably outraged by the state's greenlighting of Phase I because they knew it killed their chances of service at least for a while. Broader-picture, Phase I's inefficient use of money is a drain on all other projects by spending an outsize chunk of the transportation budget; even if we can't necessarily trace specific project delays or cancellations to it does not mean it was harmless to other projects.