The problem with a lot of European fish is the water temperature. They need cooler or in some cases cold periods that you normaly cannot provide in your home. Some need permanetly cooler water. Some people manage to provide these temperature in their basements but even there its often too warm on the long term. For example Perca fluviatilis needs winter temps below 6°C / 43 °F and can't stand above 20°C / 68°F for longer time periods.
Is that a generally accepted set of temperature parameters...or is there much disagreement on that?
Perca flavescens, very similar to your Perca fluviatilis, are widespread in North America, including here in Canada where they live under thick ice for months each year. Water temps are in the 35F range in many instances...but the species lives for years in basement tanks that never get below 55F. I had several Perch in a tank in my parents' basement when I was a kid, and that small group was installed when I was in the first year of high school (grade 9) and was still going strong when I graduated from grade 13 (I think it only took about ten years...

). I doubt that water ever got colder than 55F, and probably never quite hit 70F, so nowhere near the range they would have experienced in the wild. They grew decently well, although not as large as wild fish, and they coloured up each spring as well, although I never witnessed actual spawning activity.
That was in extreme southern Canada, but we still had solid ice each winter for a few months and the fish saw very cold temps in nature. I am fairly certain that our winters there were somewhat harsher than those in much of Europe; I'm positive that the winters where I live now are much harsher than anything Europe has to offer, and P.flavescens thrives herel..but
surviving extreme cold is not the same as
requiring it. I'm sure that they would have suffered if kept at 70F room temperature year round, although they certainly live through several months each year at that temp or higher in the wild...but I don't think they need those extremes either.
I think that in many cases, other problems arise with the keeping of native fish, some of which quickly or eventually result in death...and that the lack of extreme cold is used as an easy and convenient excuse.
Goldfish can easily survive under the ice each winter...but the vast majority of captive goldfish spend years or decades at room temperature. Different species, of course, but an example of the same idea. My Perch weren't kept in a species tank; they were in a "bait-shop community tank", as my Dad called it, and lived with Pumpkinseed Sunfish, Bullheads, Emerald Shiners, a succession of Bullfrog tadpoles and other assorted local catches. All those species seemed to do well without freezing temps.