Yeah, I guess thinking about it, that broader issue here is that, for good connectivity, you really want the metro services to run as far as a station that a reasonable number of longer distance services call at. And you see that on other lines - with metro all/most-stops trains from various London terminals running at least as far as East Croydon, Woking, Bromley South, or Sevenoaks. Other examples are less clear though: Elizabeth line terminates at Shenfield, where medium distance trains call, but the faster ones don't stop until Chelmsford. And Watford Junction - served by LNWR but not significantly (at all?) by Avanti. That does gives an argument for running EL as far as Reading.
Journeys like Twyford-Southall and Reading-Hayes provide another argument, but, how many of those kinds of journeys are there, compared to people wanting to get into London as quickly as they can?
The counter-argument is that there is only a certain level of service that stations like Twyford or Burnham can really provide passenger numbers to support, and wouldn't it be better to provide that service using a good frequency of regularly spaced GWR trains that give quick journey times into Paddington, and the option to change somewhere like Slough to a very frequent metro service for the people who want intermediate stations closer to London - rather than a less frequent fast service intermixed with a less frequent stopping service that's great for intermediate journeys, but not so great for the majority of people who want to go to London?