I think that if indeed Rabbonim rail against it, it may not be about the food itself – nothing would seem inherently wrong with the food – but its about the attitude in general that when something new crops up, people just get ‘farkoift’ to it so quickly.
There is a concept of not going for the latest fads in goyish society. Even if the item itself isn’t technically problematic halachically, we still don’t look for new distractions coming from the outside world.
Sadly, in other aspects of life, we see how peoples eagerness to invite and welcome new fashions has changed Klal Yisroel so much over recent years.
So whether chulent and kugel were at some point introduced to klal yisroel in the same way – i.e. as a local fad that was going round – is irrelevant. Right now they are not new fashionable things and therefore they are ‘just food’. Sushi will also be like that in the future. But right now, it is a little discomforting to see how a food fad that hits the goyish world seems to have the same appeal and speedy assimilation in the Jewish world.