> I'd love to have more startups like this to watch,

There have been plenty of supersonic commercial jet startups-and-shutdowns through the years, to the extent that aviation people shrug when a new one appears ( but are quite happy to take the promotional merchandise that is handed-out at airshows! )

One of the most recent prior to Boom is Aerion:

http://www.aerionsupersonic.com

Prior to that we had HyperMach who were proposing Mach 3.5

https://www.flightglobal.com/news/articles/paris-picture-hyp...

Or fancy a catchier name? Here's Spike

http://www.spikeaerospace.com/

Even the big-boys have dabbled in the arena; Dassault, Gulfstream, Lockheed-Martin, Sukhoi and Tupolev all had projects but none made it to hardware.