Well, the top sections aren't sold separately, for one thing. Another is that the 'standardization' is not, plainly, 'standard,' and a difference of .1" in the screw-post spacing is sufficient to make it impossible to use a top that doesn't come with a matching bottom.
That said, I have used new tops on older bottoms, with the result that I have at least two separate bottoms with no tops. The reason is that the curve of the feet must match the curve of the carved top exactly, and it's a pain to fit a new bridge when all you need is the top (breakable) section.
If you really want a problem with fit, try finding new tuners for a 1920s Gibson A-model. The modern spacing is off just enough that you can't use modern pegs, and I don't know of a source for ones with the proper spacing, other than really vintage instruments or parts which sell for astronomical prices. Sigh.
The difference between adjustable and non-adjustable bridges can be more than just the fit, too. There is a theory in bridge-making that says that you should NOT have a 'direct path' from string to top. If you look at a violin bridge, for instance, there are holes and 'partial' holes (the two outside strings are over holes with missing sides) under each string. Solid mandolin bridges without holes sound different from ones with holes. The adjusting screw kind, of course, have that gap under the top section that acts the same way. I experimented with the bridge on my flat-iron-style Mid-Missouri Mandolin instrument, and prefer the bridge with holes I drilled over the same style bridge without holes.
In any case, there are a lot of choices in bridges, some probably as expensive as a bottom-line instrument as a whole. I have several mandolins, from the old Gibson to a Harmony to the Mid-Missouri to a "The Loar" F-5 (no, not a "Loar F-5," but one by the Chinese company that's making pretty good copies of the "real deal"). I have mostly adjustable bridges on them, but may try a solid bridge on one or more, once I can afford any bridge at all. And I'll keep experimenting to see which combination fits which instrument.
And I'll also keep looking for old-gauge mandolin tuners, since the ones on the Gibson are starting to slip too easily.