I feel like New Southgate is the Maidenhead of Crossrail 2. On paper it's terminating there, but there's no practical reason to actually do so when it comes time to run trains. It's all about presenting the numbers correctly so the project gets the go ahead. Then, when it's halfway through and it's too late to go back, the bolt ons come out.
If Crossrail 2 goes to New Southgate, why wouldn't you extend some of those trains to Welwyn to take over from Moorgate services? New Southgate isn't a hub worthy of being a terminus and never will be. I know the reason for it going there is to build a depot, but there will be a lot of pressure to extend it from those north who want the shiny new trainline to come to them.
Provided the correct option is taken of building Crossrail 2 to serve Alexandra Palace and Turnpike Lane (both crucial interchanges) - as opposed to Wood Green to fit in with Haringey Council's vanity regeneration project - then any commuters who did want to change at Alexandra Palance for trains to Moorgate would have the opportunity to do so with regular connections onto Hertford-Moorgate services.
This then frees up the Moorgate branch to provide a more intensive service on the Hertford Loop, which needs more trains as they get very full in the peaks (though the 717s will help).
Back to the original question; it absolutely needs a deep clean and an update. I'm surprised the DfT have not required franchise holders to do something about it; that's the only way it's going to happen.