While I'm not an expert for sure, my guess is that in the old days many fountain pen users stuck with certain brands of ink. At least in the area I'm in, admittedly a rural area, people used mostly Waterman, Shaeffer Skrip, and Parker. I base that on the kinds of used, empty ink bottles that can be found in the antique shops in the area. Not totally scientific, but probably representative.
The production of those inks may have remained fairly consistent over long periods of time as corporate pressure to cut costs may not have been as strong. But also, inks known to be damaging to fountain pens such as traditional IG inks, were not used, and for permanent documents people used dip pens.
These days there are many small brands of inks on the market and even ink from foreign countries. Which may not have been so readily available in the past. And while there many be ink with ingredients that are harmful to pen components, it's not possible for the pen user to know what those are since such information is hidden in the proprietary ink formulas. And that make the mixing of inks even more hazardous and uncertain. While such mixtures may not create a sludge or sediment, their chemical interactions could perhaps be more damaging to pen components than the individual inks. And you wouldn't know about in until long afterwards since the damage may well be cumulative over time.
It would seem that the ink manufacturers should at least provide a bit more information to guide end users who are making choices about inks based on color, flow, interaction with paper (bleed through, show through), etc. as anyone can see in the various reviews. Perhaps information that a particular ink might be more suitable for a pen with converter rather than piston filler.
Anyway, just some random, silly speculations, late at night...