Not sure on the loss of the 599 but I have had an idea for an alternative similar service which perhaps Dan may consider now on a commercial basis, certainly a worthy of a trial

598 Sunderland to Hebburn via

Chester Road Uni and Hospital
Pallion Metro
Southwick Green
Fulwell Grange
Cleadon Village
Ridgeway
Harton Moor
South Tyneside District Hospital
Simonside Estate
Bede Industrial Estate (Metro / Barbour Factory)
Jarrow Bus Station

Then the opposite circular route of 599 around Jarrow/Hebburn

I was considering this on the opposite half hour to the 599 to give a 30 mins service along parallel sections.

Would have to be dedicated E200 route due to 10ft low bridge at Bede / Barbour

This opens up following links

Fulwell / Southwick to Royal Hospital
Cleadon Village / Ridgeway to South Tyneside Hospital.
Linking South Tyneside Hospital with Sunderland Royal Hospital
Bede Industrial Estate to all areas.