This isn't quite true. There are three state-backed clubs in football. None of them have won the Champions League and non-state-owned clubs like Bayern, Liverpool, Madrid etc (and currently Arsenal!) have managed to compete with them in recent years. United's transfer spending since 2013 is on par with City's and our wage bill is the largest in the history of the Premier League.
United's biggest problem isn't a lack of revenues, it's a lack of infrastructure spending and a lack of competence at senior management levels across the club. Those two issues would definitely be addressed by any perspective new owner, including US-based consortiums/PE houses. As long as we have a proper strategy and good people in key roles, we can compete in future with City/PSG/Newcastle, even without a nation state backer.
United's biggest problem is the price tag.
The people buying United would expect an ROI of at least 4% annually on their investment if it were invested elsewhere.
That number is not realistic if the price of the club is over 4 billion.
The Glazer family got 1.2 billion over 17 years. About 70 million a year.
To get 4% of 4 billion, you need 160 million a year.
The numbers obviously get worse the more expensive the club gets.
This is why it is not realistic to get business people to buy United. The numbers just don't work as an investment.
Even if they doubled the net income, it wouldn't work at 4 billion.
I'm not a financial analyst and the numbers is not entirely accurate because it will likely be more money going out
towards the end of their ownership. I think it would be a hard buy for most investors.
Edit: This is also assuming no loans are taken to buy United.