I do find it quite stark comparing the Elizabeth Line (EL) to Thameslink (TL), they are indeed essentially the same thing (RER) but the EL is far more well-known, well-used, and simply useful for journeys for Londoners than TL, which is arguably designed for non-Londoners in Hertfordshire, Kent and Surrey to get into London. It's particularly woeful for South Londoners. It should be a quick, direct, high-capacity route, but poor connections to other lines and awful-30 min frequencies mean most people I know who live near the line don't even consider it an option and instead bus to the nearest tube. Such a waste. It feels like a wasted opportunity that TL wasn't made more EL like in its central sections.
More trains should have been sent via Elephant & Castle than London Bridge, with new stations at Walworth, Camberwell, giving them, Loughborough Junction, Herne Hill, Tulse Hill etc metro-style frequencies into Central London. A Jubilee-line connected Southwark station could have been built rather than the cross-river Blackfriars. Streatham Common could have been re-built at the junction allowing rationalisation and interchange with the Balham-Norbury line etc.
Perhaps now with the reduction of daily commuting from the home counties into London, there is indeed the spare capacity to increase the London stations and frequencies of TL to make it better resemble its purple cousin and vastly improve public transport connections across the middle of South London.