I recently championed my first 5* (Bolt, Widow, Strange), and immediately my PVP changed. It's a sea of Panther / Thanos, or Gambit / Bolt, or other combination of championed 5*, most 10 or more levels higher than mine.
Maybe it's just the way the boosted lists have worked in the past few weeks, but my 4* (the highest are at 330 or so) don't last long enough; to stand a chance of winning against anything other than seed teams I have to use my 5* and a lot of ISO for skipping.
So, yes, you'll be stuck using Thanos / Panther in PVP. But PVE will be a breeze.