Chris Evans was not a bankable star before Captain America: First Avenger.
Chris Hemsworth was not a bankable star before Thor.
Tom Holland was not a bankable star before Spider-Man: Homecoming.
Chadwick Boseman was not a bankable star before Black Panther.
Chris Pratt was not a bankable star before Guardians of the Galaxy (The Lego Movie *did* come out the same year, so perhaps this one is squishier)
Some of the worst MCU movies were built on the backs of 'bankable' stars. This isn't the issue.
If it's about 'which of these relatively unknown leads since Endgame could hold down a trilogy', then there have been some real missteps and tragedies there, not all within Disney/Marvel's control (Boseman, Simi Liu, etc.). Of all the post-Endgame leads, I personally find Iman Vellani the most charismatic, but how many people even know that's the name of the actress who plays (played?) Kamala Khan?