I think Marvel made more stars than they hired.
Of the initial six, RDJ was by far the biggest name. But he was also the biggest risk. Few people doubted he could make a really good Tony Stark, but nearly everyone doubted he'd stay out of trouble long enough to do it.
Then Scarlett Johansson, who was actually their second choice after the (at the time) less well known Emily Blunt, was a big star in her own right. But she wasn't exactly known as an action movie actress. Evans and Ruffalo were left field choices, for different reasons (Evans had already played Johnny Storm, Ruffalo was known mostly as a romcom guy).
I understand them going back to Natalie Portman, because Jane Foster is a known character, to both comic book readers and movie audiences. And I understand her wanting to revisit it, if she felt there was unfinished business. It's not what I would have chosen, but whatever.
I do hope they continue to do creative things when casting movies, and that seems to be what's happening with Phase 4.
Also, I still hope that they decide to recast Katherine Langford, after not using her scenes in Endgame, and have her play Kate Bishop. It's a shame that Renner is such a dud, because they look to be basing that show on Matt Fraction's Hawkeye series, which was a lot of fun.