Just so you are clear, you have to dispose of any post-sample machineguns BEFORE you let your FFL/SOT expire. If you still have them after your FFL/SOT has expired, they are now contraband.
Also, if you get the SOT and can get get an LE demo letter, why would you want to get one of those conversion devices? Why not just get a G18?
No. You need a demo letter if you are an 01/03 FFL/SOT dealer. You do not need a demo letter if you are an 07/02 FFL/SOT manufacturer.
Well that answers your question on why someone would buy a conversion device instead of a g18. No need for a demo letter because you are the manufacturer.