For the longhaul/leisure/backpacker market, the ease of connection and the availability of lounges are not really an issue. Price, and not having to collect and re-check luggage at LHR, will be the main concern..

Connecting from an EI flight on a through ticket (and not just to BA - you can connect on to MH, UA, AC, CX, and others on a single ticket, even if codeshares are not available) at Heathrow is, in any event, not such a big deal - you stay on board and then take the bus from the back steps. I presume this will continue after the move to T2, and after BA's DUB flight moves to T5.

In any event, a lot of people buying BA tickets won't even know, realise, or care that they are being presented with options, some of which that are EI metal, some which are BA. They'll probably be guided more by price and timing.