> Writing unit tests where needed makes you more productive in the long run.
Debatable? It has positive effects for organizations and for the society, but from a selfish point of view, you gain relatively little from writing tests. In your own code, a test might save you debugging time once in a blue moon, but the gains are almost certainly offset by the considerable effort of writing a comprehensive suite of tests in the first place.
Again, it's prudent to have tests for more altruistic reasons, but individual productivity probably ain't it.
> Writing in modern languages makes you more productive.
With two big caveats. First, for every successful modern language that actually makes you more productive, there's 20 that make waves on HN but turn out to be duds. So some reluctance is rational. Otherwise, you end up wasting time learning dead-end languages over and over again.
Second, it's perfectly reasonable to say that Rust or whatever makes an average programmer more productive, but it won't necessarily make a programmer with 30 years of C++ experience more productive. This is simply because it will take them a long time to unlearn old habits and reach the same level of mastery in the new thing.
My point is, you can view these through the prism of rational thinking, not stubbornness. In a corporate setting, the interests of the many might override the preferences of the few. But if you're an open-source developer and don't want to use $new_thing, I don't think we have the moral high ground to force you.