It's worth remembering that bus deregulation also had a much less obvious effect than just the reduction of feeder/interchange passenger flows - bus services were able to start competing with Metro directly.
It didn't happen too much at first, but really ramped up in the late 90's/early 00's, especially as operators were all consumed into the "big 3" we have in the area today, and they started heavily pushing their own multi-trip tickets and fares which often undercut Metro. As Metro aged with minimal investment, some of the bus routes were even upgraded and made more "premium" with features such as AC, leather seats, WiFi and even tables to capitalise on the fact that Metro was becoming increasingly run-down and unreliable.
This will be a big part of if MtW actually succeeds - if the bus services are allowed to continue competing rather than complementing the service, it will be hard to get passengers to switch. This is also where some design aspects of the 555s bite, as the other group you need to convince to switch for passenger numbers to go up are motorists. Asking car owners to switch to a less convenient mode of transport is already an ask, and it really just puts the final nail in the coffin when there is a substantial drop in comfort too. This is especially critical somewhere like Washington.
In fairness, there's not really any scope to really reduce the competing buses in Washington without making the service worse.
Stuff like the 56 needs to exist anyway and curtailing the thing short at Wrekenton doesn't benefit no-one. Similar with the X1, you can really expect someone from Houghton to travel 30 minutes to Washington South to then connect to a Metro. It's just long and inconvenient.
Now if there was a Metro station at Penshaw, that could be different.
It's much the same towards Sunderland, you can't just not serve the hospital or miss the huge business parks to the North of Sunderland. It's why I can't see the numbers being high at all, why bother getting the bus to Sunderland, to the Metro, to change to the Metro - you might aswell stay on the bus...
That's before discussing the loads in the other direction to Concord / Washington Galleries who can only access the Metro by travelling 20 minutes in the wrong direction since it misses most the real population in Sunderland.