That's not a pound for pound argument. During the past decade United actually won things, often outperformed clubs that have also spent a lot such as Chelsea or Liverpool. Now it's fair to say that we expected more and that we wanted to see the club higher in the rankings but United are pound for pound far from the worst. In fact in England the undisputed winners are Everton, they spent a lot, won nothing and have steadily regressed to relegation contention.
Honours over past decade:
United - FA Cup 15/16, Europa League 16/17, EFL Cup 22/23
Chelsea - Premier League 14/15 and 16/17, FA Cup 17/18, EFL Cup 14/15, Champions League 20/21, Europa League 18/19
Liverpool - Premier League 19/20*, FA Cup 21/22, EFL Cup 21/22, Champions League 18/19
Let's not sugarcoat it - over the last decade we are miles behind Chelsea and Liverpool that both won the league and the Champions League. In that period, our best trophy still is the Europa League under Jose or LVG's FA Cup. I don't know if we often outperformed them, but there is no discussion that we have won much less than either of these teams.
So yeah, pound for pound we've been absolutely underwhelming and among the biggest underperformers in the league. I agree on Everton though, it's quite impressive to invest so much and remain so terrible. Honourable mention to Spurs, too, that won nothing and will win nothing, yet they tried to market themselves as a top club, did a big stadium investment and also had some terrible business over the last decade.