In all honesty, early plexi circuits were meant to be clean amps with high headroom at high volumes. They were indeed adoped widely for their overdriven sounds at even higher volumes, yes, but what they were meant to be was clean.

And in terms of why spend 3000 to mod an amp - bright cap mods are very popular in plexi amps, and many of the amps second hand are modded this way. So popular in fact, that clone manufacturers like Ceriatone include those mods in their model from the factory.

In addition this mod is so easy to do that not doing it because it's an expensive amp is a strange argument. So what it costs 3000? It's a lot of hardware, transformers and such. Not like this mod will ruin it or make it worse. Completely reversable as well.