The idea was that services would churn at Heathrow. Folks jump off, new folks jump on heading west, semi-fast to Slough, Maidenhead and Reading - at least initially. Maybe one day, Didcot/Oxford and Newbury. The existing services would remain best for existing through journeys.
It does complicate a very delicate operation. Keeping reliefs helps that (more trains drop off the reliefs at Hayes than the fasts, if HEx remains).
If HEx left, as I understand it, those paths might be taken up by either fast shuttles to Reading/Newbury, or new services via Basingstoke. However, the HEx services do literally protect the Slough stop on the Oxford fasts, which would be greatly missed in both directions. A lot of jobs in Slough.
Maybe a return to the Staines plan?