Note: What I write below is with regard to original/antique revolvers only!
I have seen plenty of Remingtons and Whitneys with bent/twisted frames. I have also seen plenty of Colt open frame revolvers with loose arbors (where they screw into the frame).
Repairing a loose arbor on a Colt is normally a very easy job that requires only a few handtools and two to five minutes and does not in any way compromise the integrity of the material in the gun.
Fixing a bent/twisted frame on a Remington or Whitney is a much more advanced repair, and could easily result in fractures in the metal.
So, would a topstrap be a good thing on a Colt revolver? Well, they obviously felt it was for higher energy calibers when they introduced the m1873. Is it useful on a open frame percussion revolver like for instance a m1860 Army? I don´t think so.
Think about this (and remember I am talking about original guns, not replicas which are more often than not put together with a total lack of craftsmanship): On a good condition Colt open frame percussion revolver which is 150+ years old, it is not at all uncommon that the wedge can be popped out and reinserted (fully seated) with mere thumbpressure. ALL my original Colts are like that. If a 150+ years old revolver, which has obviously been used (though not abused) and which has obviously never been repaired or messed with can be in such a sound shape as far as the loadbearing wedge and its fit is concerned,
does the design really need improving upon?
I don´t think so.
Regards!
Anders Olsson