The $wgFoo = $wmgFoo hack was needed for extensions because InitialiseSettings.php would run before extensions were loaded, so the extension defaults would overwrite the wanted values. But extension.json doesn't unconditionally set defaults if they're already set, so we don't need this hack anymore!
Example change: https://gerrit.wikimedia.org/r/#/c/237686/
This will reduce the amount of globals we use, and hopefully will help a little with T103886.