My concern is not whether they would be better Presidents than Trump -- they clearly would be as its an exceptionally low bar to hurdle.
My concern is losing to him again by nominating someone who cannot capture some of the states key to a Trump victory. They need someone who can win the prior blue states, plus some combination of Penn, Ohio, Florida, Michigan, Wisconsin, etc.
Biden could do it, I believe. Some of the others mentioned above, no. For example i do not think Warren could get it done. Nor Booker.