Delta would probably argue that they schedule their older frames on this route because of lower yield [0]. And you're right — political toy-spitting is one way to try to influence the decision away from the pure economics of the route. It worked with BA, it might work with Delta too.

[0] Older frames aren't necessarily less reliable — these issues could have happened to a brand new frame — and if maintained properly they are certainly not less safe. But they can be more economical in some ways (they're probably paid off), and higher-yielding destinations will reject the older frames since competitors run new aircraft on the same routes. There's not much competition on JFK-ACC.