I wholeheartedly disagree. He is the Senate Majority Leader, which allows him enormously expanded power to decide what does and does not happen in the Senate, even for things with overwhelming support under other circumstances.

Not really. He needs a majority of the Senate for any action he takes.

If the Senate Majority Leader should only consider what is best for his own constituency, should the President only consider what is best for those in states that voted for him? Or, if you prefer, should he not consider what is best for people in Puerto Rico or the Mariana Islands because they cannot vote for the President?

Apples and oranges.

If a politician has power over people, then it is their duty to consider what is best for those people in their decisions.

He's not responsible to anyone outside of his state. They have their own Senators they should appeal to.

Nancy Pelosi, for reference, gets this.

He does plenty of things that are aimed at helping people in other districts. Pelosi doesn't listen to people from Alabama who criticize her.

You're confusing "not caring about the opinions of people who aren't part of his constituency" with "not caring about people who aren't part of his constituency."