Collectability of anything is dependant upon a few things.
Low production numbers, desirable features, & nothing else like it available.
The older guns, (Flattops, Flatgates, etc) are not low in production numbers until you see a "variation" such as the 10" barrels. Add the fact that a lot of them get abused, lost, destroyed, customized etc,, then the numbers of them in good shape suddenly become "low." Then add the desirability feature,, as they are great feeling shooters,, and have been used as custom platforms,, (can we say converted to 44 spl?) and you have created another reason they are desirable. Then add the fact that the action design changed dramitically in 1973,, & now they are no longer available. Instant collector demand.
Or,, let's look at the Hawkeye. Approximately 3,300 built,, & were very slow sellers. Weren't very attractive to shooters,, as the design was ahead of it's time. Stop production,, and add the fact of Rugers becoming a collector world sleeper,, they have steadily risen in demand & prices. Nowadays,, almost all collectors either have at least one or want at least one "just to have" as they are now considered one of the premier collectables in the Ruger line,, even though there are a lot of other guns with lower production numbers.
Then the maxi,, a nice NM,, BUT built totally different than other Rugers in that the mainframe is larger. About 11,000 built,, but the program got scrapped due to a few issues with topstrap flame cutting by a few hotrodder reloaders using light bullets at warp speeds. Then custom gun makers found they could use the frame to built other odd calibers,, so the demand gets high quickly. Prices have risen accordingly.
So,, what to "collect?" for future investments??? A hard question to answer.
I recall a budding collector who scoffed at the "new" Bisley gripframe design,,, and the "odd" 32 magnum. He figured it would go away quickly,, so he aquired a nice Bisley 32 mag, to put away. As it turned out,, his variation did become scarce,, just because he got lucky & got one of the lower production types,,, but the Bisley grip frame has become a desirable feature,, so it has remained. And the caliber, 32 mag has become a popular standard,, as noted by the volumes of shooters who enjoy it. It now has a big brother in the 327 maggie,, so who knows. That guy was "wrong" in his assumptions,, but got lucky that the peticular variation was a lower production version.
Buying to collect while speculating on future rarieties is risky at best.