HEX won’t be going anywhere IMHO. The paths it uses aren’t any good for the Elizabeth line and GWR won’t want them either. HEX serves a market and with the Ł10/30 day tickets I’m back to using it over a crowded and slow EL service.
And it's only Ł6.65 with a railcard. I, too, have been buying advance HEX tickets whenever I book flights out of LHR.