Apparently TfL don't think people are switching from the tube signifcantly and most of the Elizabeth Line passengers are new, switching from car etc.
That's a considerable "The Emperor has no Clothes" invention, to appeal to various political factions. It doesn't have a lot of car switch, for the reasonably obvious reason that the last 15 years or so have seen a substantial attack on car usage in Central London, and there was little car usage into there anyway to switch. Most of the central London traffic nowadays is made up of commercial vehicles. I don't know anyone in my locality who has switched from car. There is quite a lot of induced demand, where the shorter time to the centre, especially now Bond Street has opened, is significant. And from casual comments I get the impression the western side has picked up a lot more than the GEML, or, since the throughout opening, the Abbey Wood line.
I would also make the point that in the first several months of Core service, it got a significantly good reputation for reliability (apart from not running on Sundays). That seems to have gone out of the window now, with a combination of technical issues, strikes, GEML weekend closures, diversions to Liverpool Street, CBTC transition failures (which many of us predicted for years would not work seamlessly), irregular intervals, you name it.