I don't think you're right. As someone who flies LH over the pond I love Jetblue. When I fly domestically in the US I try to get onto B6 or Horizon. I'd love to get Jetblue connections onto the same ticket. Note that we're not talking about transcons here, I love to take LH or another sane longhaul airline as close to the point I want to get and then a _short_ connection to exactly where I want to go. The B6 product is perfectly fine for that, and for sure a lot better than United RJs or E- in the quite likely case that they've already filled up F with upgraders, especially if you're on a discounted business fare.
I don't mind flying even E- on the short flights when I know I'm in J/F on the long ones either. But it is all about setting expectations. Imagine an uninformed passenger (which most are) coming off their FRA-JFK flight in J/F and then heading over to their connecting gate in a different terminal. There's no lounge when you get to the gate area, which is very confusing. Then, no pre-boarding. And you get on the plane and there isn't even the fake intra-Europe J with the curtain and the blocked seat. It won't fly for the high revenue passnegers.