I think what’s missed here is that the point of services to Reading, and hypothetical services to Didcot, is not that someone is taking a train all the way from even Paddington, let alone Abbey Wood, and instead they would take a fast train as far as possible.
Most people would be taking the line from an intermediate station to Reading or Didcot. The journey from Didcot to Reading is shorter than that in the central section, with far more easily accessible, and free, toilets at either station, and most inbetween. Therefore the lack of toilets would be less of a problem than the central section or most of the underground. It is currently quicker to take a fast train to Paddington and then a train to Ealing Broadway just about from Reading, and therefore no one from after would feasibly be making a journey longer than this, and so as much as the lack of toilets isn’t good, it wouldn’t be made worse by an extension to Didcot than it already is.