I think the more interesting missing piece of it all is a link from Clapham Junction to the Kent lines, but it avoided CJ and went directly from Olympia to Bromley South.
Today there are far more stops (Imperial Wharf, West Brompton, Shepherds Bush all didn't exist) - which ostensibly now require a metro service between them, as LO is often used - and all are well-used, so skipping some on a regional service isn't worth it. Shepherds Bush would win out over Olympia today also, if there was one call.
Then again, it's another TL Core, so everything in theory should call for pathing and then disperse to wherever. In this case, without a link to the fast lines (Gatwick/Brighton would be good for 1tph, South London locals would need 2tph+) - and only with 1tph, it's better to turn it at CJ.
Same with Watford frankly, better having a path back to a 12 car service into Euston. And in this case, it helps EWR get into MKC from opening.