Yes of course IAG will continue to squeeze as much traffic through LHR as they can, that's their model. But having a secondary hub available in DUB (and with the benefit of US pre-clearance to boot) gives them options and a safety valve in the longer term especially if LHR R3 gets kicked into the long grass again.
The assumption appears to be that Pre Clearance will only ever be at Dub/Snn......................... not one I would bet on.