"several billion pounds" was spent on many things, and getting direct trains from Littlehampton through the was an infinitesimally small part of the total. Whilst you might not have seen past that part of the TL2K sales pitch, the main drivers were to enable much increased capacity through the core, roughly doubling the original TL flow, new high capacity trains to replace 30 year old class 319s, that individually doubled their capacity, a renewed London Bridge station, reinforced traction supplies, longer platforms etc.. In other words, "several billions of pounds" wasn't all about Littlehampton through core running.For example Thameslink 2000 was sold with direct trains from Littlehampton going through the core and they now only going to London Bridge, which is returning things exactly where they started before several billion pounds worth of public money was spent. The country outside of cities need a subsidised railway unless you want to increase the divide between cities and elsewhere.
Now there are some who decry the changes to the TL services and the trains, especially here, but it doesn't alter the fact that the the "commuter class" still needs to get into London and home again*, and the effects of a very large investment have generally transformed the network for the better.
* The TL programme was completed before the unplanned impact of COVID-19 came along.