Going to agree with @Eldente that the main purpose of the Big Imbalance Update isn’t the individual nerfs, which are almost a distraction really — it’s the systemic relative weakening of all older 5* and all 4* heroes
Free “costumes without costumes” boosts for new, already bigger heroes.
But ya gotta pay (or get really… “lucky”) to get costumes for older heroes, and those costumes often do little more than provide a bonus similar to the freebies the newer heroes are getting, never mind narrowing the gap at all.