There are enough billionaires in the world to support them if they have a product. Conventional business jets only offer more flexibility with little speed advantage beyond bypassing security.
The flexibility is worth more than the speed for most flights, and not many of the world's ~2000 billionaires are going to put down a large percentage of their net worth for a fast-depreciating personal transportation device that only becomes worthwhile if they need to leave their continent.
Fractional ownership works for the likes of Netjets buying existing aircraft types, but it's another niche-sized market and essentially a way for the superrich to avoid buying aircraft.