I wondered about this, too, since I just took the plunge and purchased a fairly pricey uke. And, like you, I also own guitars and other such instruments, and like to think they will hold their value.
I'm guessing uke pricing won't go down that much. Has there ever really been a crash in prices for any instrument? Over the years, I have purchased mandolins and banjos and what have you, and the good ones are always expensive. More expensive, in fact, than I expected. A premium banjo or mandolin will set you back $4000 or more. So by comparison, ukes are still pretty cheap.
The uke "fad" may wane, but good, solid wood instruments will always be in demand. Particularly since premium tonewood is only getting more scarce. For example, my Taylor guitar has increased several hundred dollars in value since I bought it 13 years ago, despite the fact that, these days, there are a ridiculous number of solid wood acoustic guitars available to a prospective buyer. It might take me a while to sell it -- assuming I wanted to -- but I am confident I could get more than I paid for it. The same should hold true for ukes. There will always be someone out there who wants a premium quality uke, and thanks to the internet, that person is now much easier to find.
Of course, this is all a moot point anyway because we buy these things to make music, not as investments, right?
The fact that they may retain some value is just an added bonus. If I squeezed every last ounce of music out of my instruments and at the end they were worth nothing, I would still think it a good deal.