I agree that it sounds preposterous because anyone who is offered a choice between Cristiano Ronaldo in 2020 and Rashford and Greenwood would have to pick Ronaldo, who is one of the best players in the world.
The problem with that is that at this age, players usually decline and in some cases precipitously. I think that the age curve in sports has shifted, which is why you get 35 year olda still playing at a high level. But there is still a decline in overall play.
The question becomes whether Ronaldo can bridge the gap between United and Liverpool right now. If the answer is yes, then taking Ronaldo and relegating Rashford/Greenwood to the bench is the right move. If not, then we would potentially stagnate them without that much of a return.