The problem of this argument is it doesn't mean that building more transport capacity is good.
But there are specific issues that the tunnels are addressing
And that's not really the point of the argument. It's not to suggest that roadbuilding is
bad, just that it does not significantly reduce congestion, which is so often the supposed justification for roadbuilding and road-expansion schemes. There's plenty of specific reasons and justifications as to why you might want to expand road capacity that don't relate to congestion, old or dangerous infrastructure, etc. Still, generally, we should be moving as many people as reasonable to public transport, which would change the focus of these schemes.
making the car psychologically far more convenient than it is now despite the actual reduction in journey time being only a few seconds.
The car fundamentally suffers from a severe tragedy of the commons. Everyone wishes they could use a bus lane to save themselves a minute, but that lane wouldn't be empty if it wasn't reserved for buses in the first place, and everyone in the bus would be as late for work as you. It's an individualist and alienating mode of transport, shame it's just so damn convenient (and profitable).
As you say, the idea of non-electrified suburban networks belongs firmly in the bin
You'd think this sort of thing would be behind us ever since we got rid of the steam tram, but, alas, apparently not.
Is more car journeys actually a bad thing?
Especially with electric cars starting to become available in significant numbers.
Yes. Passenger cars are the biggest source of pollution in the UK, and as I've explained, the grid
physically cannot handle the electrification of just
current road traffic without a power generation and transmission capacity expansion that would utterly
dwarf even the most ambitious of rolling rail electrification programs. Rail just uses
much less energy than towing 2 tonnes of car around with every 1.6 people, running in stop-start traffic on rubber tires, whether those 2 tonnes are battery or ICE. The absolute last thing you want is more car journeys.