You’re absolutely right about frequencies to some of the extremities, however on some of the lines the trouble is there’s nowhere to terminate trains short.
Take the Northern Line. On the Edgware branch you have Golders Green, however the middle platform there is valuable for doing things with the service - for example sorting out the order of trains, putting aside an early train, changing over a defective train, etc. It’s already used in the peaks, during which time recovery is pretty much impossible as a result. Then there’s Colindale, which by the time you’ve turned a train there it might as well run to Edgware. Similar on the Barnet side, only Finchley Central offers a reasonably viable turnback opportunity, but again removing your prime recovery facility for that branch, and introducing a flat junction. Down the south end you only have Tooting Broadway, and again by the time that’s used one might as well go to Morden. Then there’s demand factors, Colindale seems to have enough flats going up to house the entire population of London, so that probably does start to justify a frequent off-peak service.
Unfortunately in many cases the reason there are these frequencies to the extremes is because it’s either the only operational option, or the demand actually is there.
There is some scope to pull back evening services, but that’s probably about as far as it’s possible to go. Weekend services are probably back to their old levels of usage in places. It’s the mass commuter peak that’s missing, so that’s probably the place to look at if trying to match capacity with demand.
But, there’s the cautionary lesson from the 1980s - start stripping capacity out, and one can get spectacularly caught out if it returns.
Ultimately IMO there will have to be a realisation that London Transport, as was, *is* an essential service, and needs to be maintained as such. I’d be quite happy for the “political” arm of TFL to be stripped out, there’s so much political rubbish in there. A return to an organisation focussed on running trains and buses wouldn’t go amiss.