I think the market is for taxi from Park Lane or Savoy level hotel, not from a bedsit in Southwark.
The Bond St exits are very handy for Mayfair hotels. I expect many would either do that, or just stay in their cab all the way.
If the difference is negligible to Airport Junction, then a Western / WRATH service could remain with 387s in theory, if rejoining the slows as planned. But the Crossrail units would be too slow. So perhaps these are Heathrow starters, but then run on to the Slough/M/T stops, or they try to serve Basingstoke direct, for instance, as well as Newbury and Didcot.
Otherwise I think it's not worth it. Cut T4 back (make the Picc also free between terminals, and Hatton Cross too), up T5 Crossrail service and go all in there. And rework the fasts timetable to take up the paths.