While it would be benefitial to Utd if they modernised the stadium, they don't need to do it from a financial point of view. Spurs matchday revenue won't be much more than Utd this season so the idea that they'll be miles behind Spurs in terms of generating revenue simply isn't true. Spurs are generating revenue in a different way to Utd but not (much) more and at a far higher cost.
United may not currently need to modernise the Stadium in terms of generating income but that misses the point. A better question would be should they of invested in a new Stadium when they were at the height of their success. Their current business model is overly reliant on filling a huge Stadium and on huge commercial revenues. Those two factors were fine when Ferguson was there and they were winning trophies year in year out.
They are now missing out year in year out on the Champions League and are currently 9th in the League.
This idea that new stadiums don't need to be continually improved and modernised is a strange one. Are you saying in 30 years Spurs' stadium will still be cutting edge in terms of facilities? Through gradual redevelopment a club can have the latest facilities at least partially around the ground every stage of the redevelopement - a new stadium is only new on day 1, every year after it is falling behind in terms of design and facilities. Who is to say that the Annie Road won't have superior GA facilities than Spurs stadium? Or in 15 years time if the club completely redevelope the Kop or Centenary, do you think the facilities wouldn't have moved on since when Spurs built their stadium?
Firstly the space to expand either the Kop or Kenny simply isn't there, they are complete non starters. The only way you could bring them up to modern standards on the current footprints would be to reduce the capacity of the stands.
Secondly I haven't said that new stadiums don't need to be continually modernised and improved after a certain time. The thing is those improvements will be cheaper easier to accomplish in a modern purpose built Stadium. A stadium that has been designed from the ground up and isn't a mish mash of old and new. You only have to look at the problems old stadiums have with joining up different sections of roofs and stands.
As I have said all along for me there is no right or wrong path and both new and old stadia have their advantages and disadvantages.