Yeah, Brightline is fundamentally a TOD operation that's making its own T. They already owned both the railroad and the downtown land, and they're owned by an investment group with real estate experience. The Orlando (and beyond) extension, and Brightline West, are in corridors with high enough demand and poor enough service (and traffic) that building new ROW can be profitable.
Would hourly-or-better service between Providence and Worcester be profitable? I doubt it. Would it be worthwhile (in terms of overall benefit to the cities)? Absolutely. But that means it will have to be publicly funded, not privately - whether by a reputable outfit like Brightline, or a fly-by-night shadester like BSR.
The actual startup costs for infrastructure wouldn't be that high; the biggest would be a new interlocking to get the trains from the P&W main to the NEC main. (BSR planned to avoid that by building their own Providence station, a dubious method that would have prevented transfers.) Some sort of accessible platform at Woonsocket (and any other initial new stations), and probably a new siding or two to avoiding impacting freight ops. There is the difficulty of arranging trackage rights, as you'd be dealing with Amtrak at the Providence end and CSX and/or MBTA at the Worcester end.