Crossrail was originally envisaged probably 50-60 years ago and started getting serious maybe 25-30 years ago for context, so this is nothing new!
Most, if not all, major transport projects in London are argued about for decades before happening in some form. As you say the idea of a tunnels between Liverpool Street and Paddington was first mentioned in 1941 but otherwise:
Thameslink - Bit different because the Snow Hill Tunnel was in place already but the North-South tunnel in the County of London plan in 1943 was a more subterranean version of what eventually happened.
Bakerloo Extension - Has been considered for over 100 years.
Jubilee Line Extension - Variation on the Fleet Line. First mentioned in the 60s and didn't open until 1999.
Crossrail 2 - Chelsea - Hackney line was first discussed in the 70s, although it has changed from underground to mainline services.
Oddly I think a lot of projects extending out of London will need further devolution. At the moment the Home Counties have transport policy decided at Westminster and don't have a TFL to drive it forward.