I'm not sure I agree with the connection between line upgrades and extra journeys. Simply because I can't see how the difference between, say, a 4-minute service and a 3-minute one would have that much influence on decisions. Sure if you replace an hourly service with a half-hourly one it's bound to attract people, but once into turn up and go frequencies I just don't see it matters that much.
I agree with the Vic Line observation, and likewise the Jubilee. However I'd say the Northern has escaped the "always crowded" situation many other lines find themselves in. Especially the northern branches. The trains are by no means empty, but it's still possible to experience a relatively peaceful and civilised journey most of the time. One wonders if this partly down to the Northern Line tending to serve nicer more affluent areas, where a greater proportion of local journeys happen by car. Even the Charing Cross branch can be quiet on a werkend compared with other lines.
There is a great deal of evidence from various modes - bus, tube, main line rail - that improved frequency drives patronage growth. Higher capacity also helps. I understand your point about what might be deemed marginal improvement but at the north end of the Vic Line we've effectively seen our off peak frequency doubled (train every 2 mins rather than 4-5) *and* the journey times are faster. That will definitely drive demand and, I think, has caused some level of transfer from the Picc to the Vic but that's just my random, non evidence based, view based on what looks to be much higher numbers of people boarding and alighting the Vic Line at Finsbury Park.
I didn't comment on the north end of the Northern Line branches. You are probably right that the demographic is more car based and there are fewer interchange options to other rail services. It's probably true on the Stanmore end of the Jubilee too. However north end of the Vic and east end of the Jubilee with more densely built suburbs and many more interchanges to other rail and bus services have seen much more growth. I am still surprised at how much local traffic there now is on the Highbury - WW Central section of the Vic. 20 years ago you'd struggle to see anyone waiting at n/b platforms from S Sisters to Blackhorse Road or making 1-2 stop trips southbound. Not now. And grappling gamely to get back on topic (!) this must be placing some extra strain on station assets and altering the business case / asset renewal needs for those stations. We could certainly do with more escalators and wider corridors and platforms at some Vic Line stations but we'll be waiting a long time I fear. The upside is that more passengers means a higher potential volume of passenger benefits which will aid any future business case.