To the 2707, it was cancelled in 1971 after several attempts by Nixon to save it.
What did for it was not an enhanced business nous, it was that wildly over ambitious spec they drew up, Mach 3, later 2.7, meaning you could not use alloys like Aluminium, it would have to be expensive, hard to work and hard to get Titanium, it was one thing to have the CIA set up front companies to get it from the main supplier, the USSR, for the small number of A-12/SR-71's, quite another for the 2707.
Part of the over-ambition, of course, was the size. Not that it ever got far enough to truly test the market, but it would have relied on a large number of passengers willing to pay a premium for speed to establish each route it would fly.
Boom does have a couple contrasting positive factors. Obviously, it is less technically ambitious: Smaller, lower speeds, 50 years of improvement in materials and design tools to rely on.
The number of viable routes is also defined by an aircraft roughly 1/3 the size of the 2707, and operating in a market that has literally grown to over 10 times the size it was in 1971.
If designing and building a new, environmentally acceptable SST, large, medium or small, that was also a commercial prospect, why haven't Boeing, nor Airbus, nor for a potential SSBJ
This is certainly a compelling argument. The only counter I can think of at the moment is risk aversion. Boeing and Airbus both operate in a demonstrated market. They know continual improvement in subsonic aircraft makes them money. Investing in an SST might not make them money, and even if they think it could, it still needs to make them more than further investment in subsonic aircraft to be worth prioritizing their investments in. Their investors are not clamoring for them to try out the supersonic market, either.
In a sense, for Boom, the supersonic market is not about risk, but about opportunity. They've got close to zero hope going head-to-head against Boeing and Airbus in the demonstrated market. So they either need to look at an alternative market, or not bother. Likewise, the investors who want to take a swing at the supersonic market won't get enough traction among the rest of the investors in Boeing and Airbus.
Hence, Boom either tries the supersonic market, or they don't try at all. I fully recognize the best answer may be that Boom should not try at all.
It's not my money though (and it's too risky for me to invest in even if they were publicly traded), so I'm glad they are trying and I get to watch from the sidelines and hope they get a win.