Although it technically has cross platform GUI, Flutter has actually reimplemented all the native OS widgets. As soon as the OS changes some look and feel your app will look out of date.
Yes it will look out of date. But whats important? A functioning app or good looking one? We all know that websites written for ie6 are out there today making millions in revenue. It always comes down to the functionality not the looks
You can be as tolerant as you want, that's not how things go for iOS apps. It's certainly not how things go on macOS except when there are no available alternatives.
Windows, Android, and the web, constantly changing so that there's never really been any sense of stability, blind a lot of developers to the fact that good design is both functionality AND looks.
When people complain about designs of controls being out of date, they don't only refer to looks. They refer to how they feel, behave — or more succinctly, don't correspond to the expectations set up by the rest of the system.
To reduce the debate down to looks vs. functionality is reducing things down to the wrong level, missing the forest for the trees, and ignoring the user — a person who often isn't able to express what they need but knows what they want, not often realising that they're the same thing.