6 encounters/ 2 short rests per long rest using the Adventuring day XP chart in the DMG isnt 'extreme'. That's the median.
If its not the median, the fault (class imbalance) lies with the DM.
Doesnt happen in my games because I use doom clocks, or simple 'nopes' to stop the 5MWD.
But I agree (and its obvious) - a long rest resource based class (Paladin, Casters, Barbarians) are going to dominate in single encounter adventuring days. Short rest based classes (Fighters, Warlocks, Monks) will dominate in longer days, featuring more encounters and more short rests.
Rogues are the outlier, in that they're rest neutral (barring HP) with most of their abilities at will or simply requiring set up.
Caster supremacy only exists when they're free to blat high level spells with no consequence. When they run out, they suck bad.
And THATS the reality. Martial/ Caster disparity only exists depending on the encounter and rest frequency meta of your campaign. If wizards are dominating, force them into longer adventuring days. If the inverse is true, do the reverse.
People totally ignore this context and it shits me to tears when it happens.