The problem with relying on post COVID passenger reductions is that whilst tube numbers are down 20% since COVID that just means they're at the level they were in 2009/10. Even if they don't bounce back fully this year, post COVID working hasn't cut out the peaky nature of tube travel. Even if people work from home Monday and Friday, they'll often all come into the office on Tues-Thurs making peaks on those days.
I agree the demand for capacity on other lines that is even more pressing. Last time I rode the tube regularly was 10 years ago and even then I recall the Victoria line during rush hour was literally cattle class, your face in someone else's armpit. The 2019 numbers put the Northern Central, Jubilee and Victoria lines are well over the 100% mark at peak time. Despite having the highest comparative fares compared to other cities the Tube is still woefully underfunded. There is so much technical debt because much of the system was built in the 1800's and early 20th century. First mover disadvantage, because we got to make all the mistakes.
I think you are really overegging the impact of the skipped station (turnham green?) You can just get a district line train and change. Ok, it's not ideal but it's probably only adding 5-10mins to journey times. It's one station on a network of hundreds and does not justify billion+ spent for it.
And I don't think there is a business case for further capacity, as not only would it require resignalling it requires another batch of trains to be built to get it up to 36tph. There isn't the demand for that post covid. This may change but there are much more pressing captial projects for tfl (bakerloo line extension for example) imo.