Net spend alone is definitely flawed, but if you take team quality at a given time into consideration, it's a pretty good indicator of how well a club operated since then. Take City for example, they HAD to have a huge net spend after their takeover since their team consisted of very average footballers and in order to build a huge team, you will need to spend a lot.
In the case of United, your squad was probably the fourth or fifth strength wise (above Liverpool and maybe Arsenal), thus in order to close the gap to City and Spurs (who arguably had the best squads back then), you had to invest a lot and thus have a big net spend in order to even be able to compete for the title (bar some freak season like Leicester had for example). City arguably had the strongest or second strongest squad back then already, they spent even more and are winning the title by a countrymile by being by far the "best" team in the league with the best squad. Spurs barely spent anything and are still doing quite well, their squad was already strong/young enough and they lacked the funds too. Liverpool arguably made the largest progress in those years though, their squad was the worst, all their star players now have either tremendously improved (Firmino, Coutinho) or have been bought for not that much comparably (Salah, Mané). They lost their star player Coutinho and replaced him with VVD and Ox who are doing okay even for their price tags, and I doubt that their wage bills have risen too much. I honestly think that bar Arsenal every top 6 club has been doing quite well within their ambitions and financial possibilities.
