There really aren't that many people interested in old sewing machines, and it seems to me that most of the people I encounter that are interested don't have much of a budget to devote to them.
Those that do have the funds seem to gravitate to the early machines were there is a much higher value as the machines are quite rare.
I think until 75% of the 1900-1950 era machines are distroyed, the value isn't going to go up. Anything that ended up being mass produced just as too much volume available.