How many one club players are there any more, in reality?
It is highly unlikely any player signed these days will stay at United for their careers. By virtue of the simply fact we sign people, it proves these people are not one club people. You will get people who stay, but mostly they will be English players for english clubs, spanish players for spanish clubs etc.
As people have said, we got 6 years from Ronaldo. we also got 3 leagues, a CL, an FA Cup, two league cups etc. Sure, it would have been nice to see him at United for all of his career, but if you could turn the clock back, would you really say no to signing Ronaldo, after everything he helped us (massively) to win?
If we sign Benzema and get 5 years from him, or even 3 or 4 years - if we are succesful in those years is it really a bad thing? Especially if he is sold at a profit after that time?
If you confine a transfer search to players you are almost certain would stay at United for the rest of their careers, it would be a very short list of people imo.
It is, imo, simply unrealistic and impractical to base a transfer strategy on such ideas, certainly in the modern game.