I think you're a little too high roller-focused... us regular folk [would take the OL] then hoofing it over a long pedestrian bridge... So would tourists staying in downtown hotels without rental cars. Also residents of Downtown and adjacent neighborhoods.
Please simply accept the fact that Wynn's business model does not require heavy-rail transit to be successful.
Is this hard to accept?
Sure, there are heavy-rail devotee's out there, but they are few and expensive to serve. Are you telling me that your "fancy suit" and "evening out" are so precisely calibrated that they would leap at an OL-to-Assembly-to-ped-bridge routing, but somehow be entirely hostile to a OL-to-Sullivan-to-Shuttle Bus (or 104, 105, or 109 bus)? Or a bus doing the last mile from North or South Station? C'mon.
Seems to me you are defining what modes "work" for a Casino waaay to narrowly solely for the purpose of imagining that Wynn should toss some commuter-peak infrastructure your way. The reality is that there are enough people--even transit enthusiasts and multi-modalists like me--who understand that some trips are special and not made by heavy rail.
Chasing these (few) heavy-rail purists with bridges would cost more than Wynn could ever expect to make back.
Some tourists might be 100% T-based when in Boston, but enough of them pay ~$30 each to ride on circulator "trolleys" to keep a whole lot of trolleys circulating. Or an Uber which will be a short/cheap/easy trip from all tourist hubs. While car-free, they are not heavy-rail purists. They're proven bus-users.
Its going to be way way cheaper for Wynn to slip a little extra cash to the current operators or start one of his own or indeed, use Uber-style tech to dispatch a shuttle to you wherever you are. You needn't be a high roller for him to give you a directed subsidiy if he's 100% sure you're coming to his property.
Or he'll just send a big black-and-bronzy VanHool bus to wherever car-free people congregate: BCEC, Hynes, Sheraton, Copley, Long Wharf. It isn't hard to predict, and a point-to-point bus has no risk that you'll wander to some other destination along the way. You're his captive from the moment you board his bus, and he'll pay a little extra for that (and still pay way less than the mortgage payment on a bridge across the Mystic or Malden river)
Meanwhile, why should we think a Casino needs/wants/serves heavy rail patrons any differently than the Chestnut Hill Mall or South Bay Center (had been for 15 years) or any Costco or Ikea or Home Depot? There are enough patrons who have cars, or who hire a bachelorette limo, or any of a myriad of shared (bus) modes that there just isn't enough $ at stake in the I'll-only-come-by-Heavy-Rail market to actually chase it with very expensive, very un-targeted heavy rail improvements.