Your post reads as FUD. Of course it isn't because I assume you are not one flutter's competitors. Nevertheless, worrying about what something might do in the future is classic FUD. We don't know if those others will develop hot reload, we don't know when and we don't know how good it will be. So FUD is a poor basis for a technical decision.
There's some merit in the mature language argument but dart has been around for some time now and doesn't have any radical features - although on the other hand you seem to be saying it's not novel enough like Oberon etc.
Do all those others have 20-30 years of libraries? I think that's just Qt. I spent a couple of years using Qt and it's very good but it is C++ which isn't for everyone. (The python version probably fails your maturity criteria)
There's some merit in the mature language argument but dart has been around for some time now and doesn't have any radical features - although on the other hand you seem to be saying it's not novel enough like Oberon etc.
Do all those others have 20-30 years of libraries? I think that's just Qt. I spent a couple of years using Qt and it's very good but it is C++ which isn't for everyone. (The python version probably fails your maturity criteria)