When a new language is created, its feature set is a product of the times it is created in. i.e. what features are considered useful, popular, best practice etc. Of course times change and with it what this set of desirable features is. Languages evolve by adding support for some of these new features. But adding these new features whilst preserving the old becomes increasingly difficult and the new features tend to be somewhat hobbled and don't fit that well with the older features. For the first 10 years or so this tends to be ok but when languages get to the 10 - 20 year mark they start looking increasingly like frankensteins monster - a mixture of parts from different places / eras, none of which work terribly well and which don't fit that well with others. At some point IMO it's worth jumping to a new language despite the smaller community and package availability. But it's often only when you do try it for a while when you realise how much better the dev experience is compared to the frankenstein languages which have on paper adopted similar features. To me this is where Java, JavaScript, Python etc are now. On paper they can claim many of the features now that Dart has but the experience using them is not that of using those features in Dart. 

I can't imagine there are many decent developers who if they really give dart a proper go, wouldn't much prefer to work with it than JavaScript.