They don't really need to make the BBJ
XL. The reason is this:
is a long range - high speed derivate of the 737. Customers normally get a "green" aircraft and take it to a complete center as Jet Aviation Basel or Lufthansa Technik in Hamburg. Both are specialized in outfitting business jets. This job is actually by far more demanding than it may look. The interior can easily cost more than the whole aircraft. Those customer normally don't ask "what can we do" they just say "I want this, this and this..." Then a team of engineers has to evaluated and develop a solution and get this certified. Imagine, building a gyro stabilized bed or a bathroom in marble with a "certified, turbulence proofed" bathtub in. The marble can't be taken as a block of stone, as someone would do it in a house. This would be to heavy and not crash proved. So they need to cut a very thin layer of the stone, that is going to be laminated on a special Honeycomb. Finally it needs to look great, it needs to be resistant and it has .... Once again to be certified for aviation use.
So for a 777XL, this plane is already fast and long range. You just take it to a complete center, ask them a team of engineers and here we go. If you add 100 million $ you well get something real nice, that will last for a while
...............or at least until your wife doesn't like any more the colors of the curtains.