Our "net spend" over the last 10 or so years is because we were shit when Sheikh Mansour took us over. We had no one decent to sell and needed a lot of money spending on the squad. By doing that, we've been successful thanks to good management, which has earned us lots of prize money and commercial revenue, which we've then reinvested fully in more good players. To the point where we're now spending what we earn.
Compare it to 2 people going into a Rolls-Royce dealer, one of who has a 12-month old Rolls-Royce and the other a 10-year-old Ford Fiesta. If they both buy the identical new model, one will have a significantly higher "net spend" than the other, but they've got cars of identical value. If they both do the same next year, their net spend will be the same.
Looking at it another way, if you get a player on a free, and pay him £300k a week, and sell an academy player on £20k a week for £10m, people would claim a negative net spend but the overall player cost has increased from £1m a year to £15m.
You can demonstrate this with actual figures from our accounts. In summer 2017, when we spent over £200m on players, with a "net spend" of around £120m, our wages and amortisation had only increased by £8m over that financial year, plus we'd made a £39m profit on sale. So we'd improved our bottom line by over £30m. Even if you knock off the previous year's profit on sale (which was £34.5m), we'd still only increased our bottom line by £3.5m despite having spent over £200m on new players. The reason is that the wages and amortisation of the players who left pretty well balanced out the wages and amortisation of those coming in. And we'd increased turnover, which absorbed that small increase in wages and amortisation.