Not enough interest, too much a labor of love and these ideas are just cool projects/ not really for profit!
Yah, by the time you buy some parts and then spend time taking away material. welding parts together, grinding to smooth things out, and then refinishing everything to look good. you have far too much time into the piece(s). You either work for less than minimum wage or you have to charge more than most people are willing to pay. You really need a production type shop to make those kinds of conversions with fixturing to allow making multiple parts in one set-up. If you need older production parts to modify you can pretty much forget it.
I probably had 20 hours in my Sterling magazine conversion. Yes, a lot of time was spent figuring out the dimensions to machine the mag well so that I got the right magazine depth and that wouldn't need to be done each time. The hand grinding of the bolt could have been replaced with a custom made milling cutter but I wasn't going to invest the time or money for that.
3D printing would open possibilities for some projects but it's not for really high strength or wear parts. For that you need steel.