Actually, I think the success of Go has proven the contrary. You can make a modern language without those features. In fact for an MVP you want to cut difficult or not absolutely necessary features from the first version. So I think Go did fine here and I encourage other language designers to follow their lead.

One cavaet is if you don't think about at least how generics might work in your language in a backward compatible way, you box in your future options. See java for how this goes poorly. If you look at the generics proposals for Go you see how this has greatly complicated the task. That doesn't automatically mean it was a mistake to do it that way though.

If you try to do everything at the beginning you might take so long that you fail outright or don't get the early adopters who help shape the language. Plus you'd be behind the curve on adoption. So I think Go did the smart thing here.