I remember after Zulu came out, there was a huge upsurge in the Martini-Henry... everybody and his Uncle Clarence wanted one. Nowadays... not so much. Not that they will lose value, they just won't experience the wild swings that 'in vogue' rifles go through.
Look nowadays at Garands and Carbines.... Band of Brothers , the Pacific, Saving Pvt Ryan... all that Hollywood BS does affect market price, at that particular moment... but never lasts. Consider the Trapdoors, Sniders, Spencers ... indeed, all manner of early breechloader or percussion ( and yes, regardless of historical context) after Burns' Civil War. for example.
I would say the intrinsic value of a milsurp will always be there, and will never 'crash' , to use the OP's metric.
Just be sure to obtain as high a quality example as you can possibly afford. We all know, on this forum at least, that a solid, numbers matching example in NRA Excellent with proper, period accessories will always hold it's value regardless of it's 'popularity' at the moment.
I reflect on a recent Arisaka that was displayed here just recently. I have never seen one in such fine condition, and completely un-Bubba'd.
I consider THAT an investment. I want that one so bad my teeth hurt...