Ive used this route a couple of times recently, loadings are generally pretty good (used on a Saturday / Monday this month).
it takes the quirky route to the KO line, offering good views of Wembley yard including a stop in the middle of nowhere (mitre bridge) to do the ACDC swap, uses the “semi”fast lines at Wembley (as opposed to the WCML fast or the slow/shared tube), and so makes a semi-fast service in place of an all stations, it is used by passengers to “speedup” a journey before hopping to slower services down the line. It also gives the North access to Westfield at Shepherds bush.
I‘m not convinced its being used by Manchester -Brighton passengers any more, or Avanti to Southern connections either, but its definitely well used, however….
Its fortunate that the track geometry prevents slow line to KO line access, doing without this semi-fast service or withdrawing it and forcing changing at Willesden Jn would be very uncomfortable for many northerly based Westfield shoppers, there isnt another way.
How comfortable is a 378 going to be on a Milton Keynes semi-fast, and how long before LO would want it to be an all stations, or drop it totally and force changes at Willesden jn ? - I wouldnt trust LO.
Getting from North London to Clapham is also a quite good timing compared to the crossing London on the tube, and with all the furniture re-arranging at Euston, giving it a bit more hype might be helpful, however conversely as Wembley Central is a narrow platform underground tunnel with high speed services through it, they close the platform between stopping services as its not ideal… so presumably thats why this service goes out as far as Milton Keynes, and makes it an attractive semi-fast into London, Watford and Wembley for other services…
So if Southern withdrew, perhaps Thameslink with a restored Brighton service to Milton Keynes would be a better option ?
I think the giant killing miss here is not getting a link from North Pole to the new Old Oak station, it would give the South a feed to HS2 without going via central London.
in short it feels like what it is, a bunch of left overs joined up into a single route, and on balance looks to be successful in what it does, and will continue after HS2.