The long development cycle isn't so bad given that they're developing it on a relatively lean budget, though I would be worried there is no pot of gold at the end of that rainbow.
People today equate luxury with cabin size or at least the perception of cabin size. I'd rather fly coach in a 757 than first in a 737. (Though the A220 is better than you'd think despite its size because the shape of the cabin is better suited to human passengers; just thinking of the curve of a 737/A320 makes my neck knot up)
Supersonic airliners are going to be super-narrowbody; it's hard to tell if people are going to prefer a shorter trip in an uncomfortable plane or a longer trip in business in a widebody. The supersonic private jet market might be better in that there is already one choice of a cramped small plane that people love because it is a little faster than conventional airliners [1], that market might take any speed improvement they can get damn the cost.
The interesting idea that disappeared in the 1970s was something that travels in the Mach 3 range which actually has good fuel economy when you are high up in the air and could cut the effective capital cost of the plane in half because you could make more flights. In fact analysts in the 1960s thought the 747 was a temporary thing that would become obsolete because of that latter factor. The biggest trouble with that is how does it take off and land, it needs much bigger wings for that for takeoff -- the SR-71 took off with a minimum fuel load and then refueled which would be non-starter for commercial aviation for so many reasons.
A "relatively lean budget" may prove incompatible with developing both a supersonic airliner, and its engines. Even Airbus and Boeing don't try to do that - and they're working in the easier subsonic realm.
Hard to say. You can get an engine to work at higher speeds than it was originally built for by enclosing it in a duct. Also you are seeing expensive and risky engine development for mainstream airliners [1] precisely because it is a mature technology where the purpose of recent developments is to optimize for the last few percent of operating cost.
The first supersonic airliner of the next generation is going to be of interest because it's the only supersonic airliner, the second one is going to have to squeeze out another 10% of fuel efficiency.
The article's premise of their full-scale version being in regular commercial service in 2029 should be regarded with appropriate skepticism.
Especially if you read about its current state of development - https://en.wikipedia.org/wiki/Boom_Overture