God, I hope not. They would have to re-implement every tiny little detail of every button, textbox, and every other widget on each platform. Then they'd have to keep it up to date every time the platform updates. That's just an impossible task.

It's pretty possible, and has been for a while: 1) Flutter already implements the Material spec and Cupertino spec, so the biggest cost, the upfront cost, is satisfied. In fact, the Flutter team found inconsistencies with the Cupertino spec vs. what iOS' animation kit implements and filed bugs for them. 2) It's OSS; this problem will minimize as more people are contributing and using Flutter.

You mean just like people are contributing and using Ionic, Cordova, Qt, Xamarin?

Most of the UI on mobile is custom, material libraries are also not that "native" and usually lagging behind the current specs.

Most iOS developers writing 'custom' UI, etc. are just customizing UIKit controls. Same for Android. Not many are creating UI controls from scratch with custom hit testing, drawing, and accessibility handling.