As I've said previously, this is utter nonsense. In a year or so's time we'll take a look at Utd's accounts and see whether their commercial revenue has grown over and aboe the rate it's grown in previous years. And then I'll tell you I told you so.
That's not the point though. A certain level of expenditure is required to allow the existing level of commercial growth to continue.
Could that same growth continue if they never bought any players? - No.
Could that same growth continue if they only spent £50m this window? - Probably/possibly in the short-term?
Does spending £100 million on one player now, mean that the same growth will continue for many years while also having to spend less in future - Quite possibly
This will have been thought about in a lot of detail and the truth is that if they can continue to grow the commercial revenue in the way they have been, then Pogba's transfer fee will be 'repaid' in a few years. The question is, could they have continued with similar same growth without spending as much money? And they obviously feel the answer is not.