In fact, it would be the right decision, another thing is that it is simply impossible for financial reasons. United (like probably any other club) cannot afford to change the team entirely when changing the coach. Therefore, United are expecting targeted changes that will gradually lead to the team that Amorim wants. As for selling/renting players, I don't think there are many options. Who needs Rashford with his salary? PSG were after him before Ten Hag.
Amorim needs a serious strategy to drive the club forward. What Manchester united is facing now is multidimensional, for instance, Manchester United has an issue with their squad composition. While there's an overload of attacking players, there's equally an issue with a limited defensive which was an issue underlayed by the past administration of Eric Ten Hag, now Amorim needs to fix that in the midst of a limited resources available to him. They must also work on their recruitment process and strategy which is something I believe Amorim have to address in the interim.
In fact, the squad depth that Manchester United currently has is quite good, it's just that they are not yet accustomed to the strategy implemented by Ruben Amorim. In the future, of course, Ruben Amorim will utilize the transfer market to build squad depth that suits his strategic needs, so that it will automatically improve the team performance to be more consistent. For now, with the available options, Ruben Amorim must work hard to encourage the players to bring out their full potential. So it is not surprising that Manchester United is still experiencing ups and downs in performance at this time, everything takes time to process.