Well, I have certainly seen reports that CO fills roughly 50% of their international seats at EWR with connections. I don't think it unreasonable to think DL will see a similar ratio. And even if they only managed say 25% of the traffic as connections, that would seem to be a critical mass worth making some concessions for.

As for transferring at T5 being "generally terrible", all I can say is that is not my experience. I would also note that FT is littered with complaints about connections at pretty much every hub, so I hesitate to draw any conclusions from that. It is not a perfect process, but I find my typical experience there to be better than connecting at AMS, CDG, or FRA, largely due to the superior dining and retail options.

I'm making no claims as to whether your prognostications will come true, but simply pointing out that, even if they did, this does not necessarily mean it won't improve the experience for most passengers. Taking your LHR example, you are talking a delay for originating passengers of between 13 and 20 minutes. While most people would love to get those 20 minutes back, they may not be willing to trade a good place to eat, adequate seating, and a modern lounge to do so.

You are assuming that everyone has the same preferences as you and I simply don't think that is the case.