One downside is that people will see this cool idea on the internet and possibly want to do something like that themselves, which creates a market that wasn't there before, competing with those who want them for their original purpose.
I have frequently seen the treadle bases with no tops at antiques/junk stores. They sometimes also have the drawers being sold with no cabinets. The reality is that many of the cabinets have gotten completely delaminated after years of storage, and most people are not going to realize that there are some who might treasure the old thing. I can't say I've ever seen a complete treadle cabinet offered without a machine in it. The triangular shape that they needed to fit the vanity into the corner means that they had to have used a different piece of wood. Probably the original top was in very bad shape, if it's any consolation to those who would have liked it for a sewing machine.