no one in their right mind would use it to get into central London
I would actually disagree with that point. That is because with most of the intermediate stations, the existing service is a slow service into London Euston. A hypothetical bakerloo line service wouldnt take much longer to reach Central London, and then runs further into central london directly, saving the need to change for the tube at Euston at present, which would make up for the marginally slower service into Central london itself.
Obviously at present though, the infrastructure on the bakerloo line is pretty bad, with old trains, outdated signals, and it terminates within central london. You would probably need new trains, given many 72 stocks have now been scrapped, and the remaining fleet are over 50 years old, so you would create plenty of operational issues on the line. And the big one, there is no functional fourth rail beyond Harrow, as its either been lifted or its far too old to ever be reactived, so that would need to be relaid.
All of those costs, just for serving about 6 stations, some of which already have sufficient services, or they have alternative express services into central london, which could be increased in frequency by adding in stops to some services (such as the LNWR Tring service) if demand ever justified it at, say at Bushey. In general, capacity could be increased on the overground, because Watford DC line/Lioness line trains used by 5 cars in length and now they are 4. If required, I am sure TFL with the right funding could order extra trailer carriages to make some of them 5 cars in length, which could boost capacity by up to 25%.
In general, it maybe could be looked at in the long term, but I think at this time cheaper options are available, and only if the stations between Harrow and Watford J actually needed more capacity.
