This has some truth to it (although perhaps not true in comparison to all other clubs).
In 2013-14, Spurs, for example, had only 42% of United's income (€216m compared to €512.6m). But now (according to the most recent Deloitte rich list) Spurs have 73% of United's income (€521.1m compared to €711.5m).
In absolute terms the income gap with Spurs has also narrowed over this time, from €296.6m down to €190.4 ... and on the other side of this coin is the much greater wage bill (etc) at United.
This is correct. There's no question that United will face challenges going forwards, as rival clubs begin to achieve something akin to financial parity.
On the finances, football clubs of course have three streams of revenue: matchday, commercial and broadcast. Our matchday revenue has been flat for a long time, as a result of ticket price freezes (approx £110m), our commercial revenues have been flat since 2016 (approx £275m) - the seemingly endless pool of willing sponsors has run dry - while our broadcast revenue has increased by £100m since 2016 (£140m - £240m). Unfortunately, our wage bill has also increased by exactly £100m in the same period (£230m - £330m).
In other words, all revenue sources are stagnant, except for the ever-increasing TV deals, which are immediately being used to pay for the increasing costs of player renumeration. Thankfully, the Premier League and UEFA continue to do a remarkable job of selling their products across the world, otherwise we'd have no way to pay for our ballooning wage bill.
I'd love to know what our current commercial partners are making of all of this. I'd say that the chances of the club substantially increasing on any of our headline commercial deals when they go out to tender, are dubious at best.