That's what they are. Iron Man, Captain America, Ant-Man etc were B-list characters before Marvel launched them into the stratosphere of popularity and mass appeal. And the reason why is because they didn't have acesss to Spider-Man, X-Men and Fantastic Four. If they had, the MCU would almost certainly look very different today. So in some respects, I'm glad they didn't have their crown jewels in 2006 as it forced them to buckle down and get creative; building a universe around lesser-known characters. Now we have a Shang-Chi and Eternals in development, and a Black Knight film rumored. That's awesome and all, but these guys are C-listers. They are virtually unknown even to the most hardcore Marvel fans minus maybe Shang-Chi (know him from Spider-Man). Now let's be honest with ourselves, if Marvel had access to the mythos of F4/X-Men in 2018 the Eternals wouldn't even be in the back of their minds.. That movie is now too far into development to be delayed but Shang-Chi? Black Knight? We know Marvel is excited to get these characters back and we know Feige's history with the X-Men goes back 20 years. We know he's excited to get into the X-Men mythos per THR. We know they're excited about using Wolverine, We know Marvel wanted Galactus and the Silver Surfer in Avengers 2-3 and Gunn wanted Kang.
The Phase 4 slate past 2020 isn't even set in concrete yet. (If they get the properties back next month, they could pitch to writers as soon as March-April if they want) I could easily see both Shang and Knight quietly disappearing from the slate to make room. Iger has made it clear that these properties will be very important to the future of Marvel.