Well SIG kits are available, just not through Tower. Reason to buy through TH for the SIG is to get easy credit, otherwise best to go direct to SIG.
I would think a viable company could be created to build kits on demand. For the most part the hardware is all the same, so would be a matter of printing plans, laser cutting the parts, packing the proper sized sheets and sticks, and having a supply of canopies and cowls. The company would need to have molds and forms to make the plastic and glass parts for each kit, but with the right planning, they should be able to put together any kit made by any company that no longer exists such as Pica, Byron, ETC. provided of course they can get the rights to do it. The key though would be to know which ones would still be popular to sell enough to offset the plug and mold costs, and the time needed to convert line to raster for the lasers.