I think we have seen history made today- two pages of folks in agreement with me!
Let's put it in perspective. There are many breaker styles that have come, and gone away. Square D had their "XO" and who can forget Pushmatic? Then there's Zinsco, and the little Edison-base breakers.
Some things do better in the marketplace than others. There almost seems to be a pattern - some firm rules the roost because they're cheap, only to fade away a few years later.
I've been involved with this site since the late '90's, and if I've learned anything, WORDS MATTER. An awful; lot of assertions seem to fade away as soon as you pause to ask exactly what was said. The FPE discussion is fraught with such innuendo.
Even if you think FPE is the cats' meow of breaker design, you're left with a few major drawbacks. Those factors alone ought to ensure a steady replacement of them, over time. Nothing is forever - not even diamonds.
I wish the OP would explain under what circumstances someone said FPE was 'illegal.' Surely it wasn't a new instal. Also, what were the consequences of finding such a panel?
Inquiring minds want to know