I think it's a natural thing to become more selective or focused with any sort of collection. It's also perfectly natural to move on if you lose interest. Once you've learned all you care to learn about any interest, and the more you know the harder it is to learn anything new, it can become easy to lose your passion for that interest and move on to something else. Not having set goals or purpose for your collection can also lead to loss of interest. I've only been playing with old sewing machines a little over a year now but am already learning where my stronger interests lie. While I love the flashiness of the dashboard era machines, I don't go out of my way like I do for the older American and European made machines. The old straightstitchers (and the artwork on the machines and their cabinets) just call to me louder than the others. I am focusing on my goal of having several hand crank machines ready for next year's fair and I also want to try to assemble a good cross section of older machines to show the evolution of the modern electric sewing machine.
As I've acquired more I've also become more selective in my tastes. Admittedly budget and space concerns play a part in that as well. I try to remember to ask myself some basic questions before I buy. "Do I love it?", "What am I going to do with it?" and "Where am I going to put it?" If I have good answers to those questions along with the ever present "Can I afford it?", it comes home with me.
I already have all the functions I need in a sewing machine other than possibly a full-on industrial walking foot for leather and upholstery (which I haven't needed yet) so my decisions are based more on aesthetics than function at this point.
Rodney