We have Fernandes in Rooney's position, who is much more experienced / more of a finished product, if you want to get particular. But the question was about whether two young, inconsistent attackers could play a role in a successful team. So Martial and Rashford are a straight swap for Rooney and Ronaldo, and that 06/07 team played some lovely football with a lot of success. Without another elite forward beside them, if we're being really critical. Van Nistelrooy had left and was mostly out of the picture the season before.
Not sure about the Berbatov point either. He didn't really improve the team so I'm not sure how that's evidence that signing another peak age player is the right strategy. Sir Alex just really liked buying forwards.