Two massive superstores are opening in North Sunderland in the coming months - Tesco at Wheatsheaf and Sainsbury's just outside of Castletown. These stores are going to be in direct competition with the existing Morrisons at Seaburn and will actually be far easier and quicker to reach by public transport.
I think the 99's days are numbered to be honest. I'd be quite happy to see the return of the 36A/36C instead - every 30 minutes Park Lane, Hospital, Castletown, Witherwack, Carley Hill, Southwick, Wheatsheaf, Park Lane. Then cut the existing 36 route to every 30 minutes. On 36 and 36A/36C combined, there would be a bus every 15 minutes between Sunderland, Southwick, Hylton Red House, Downhill, and Town End Farm. This is down from every 12 minutes at present, but I think a 15 minute frequency would better match demand. A lot of people were upset when Go North East withdrew their direct service from Witherwack and Carley Hill to Sunderland, but this would return that link as well as retaining existing 36 route on Ramsgate Road which has no other bus service. Perhaps the 36 could even run to Boldon ASDA instead of Town End Farm, providing faster connections with X3 to Gateshead and Newcastle.
Regarding the Seaburn section of route that would be lost, I think it's about time that the 700 was extended up the coast from St Peters to Seaburn. It already goes as far as Roker during the summer, but I think a year-round service up Dame Dorothy Street (which again has no other bus service) and up to Morrisons at Seaburn would be very well received. St Peters Campus could be served in both directions - there is a way around the one-way road at St Peter's Way that Redby once took advantage of.