So now your Wizard is better at fighting than the Fighter and better outside of combat by a mile, why even have the Fighter class then, so they can carry your extra spellbooks?
Yes and the game is better for it. Most players playing Fighters like it more, find it more immersive and more fun and most players playing Wizards like it more, find it more immersive and more fun. Players of all types are more attracted and by more product.
It works for everyone.
You have the Fighter class because people want to play a Fighter, if you don't want to play a Fighter because you can't be as powerful as others at the tame then don't play it, but don't bash us that do enjoy playing the class.
D&D is a team game and if the best thing for the team to win is to have my Fighter carry the Wizard's spell books then that is what I will do. I never had that happen in game yet. I have carried loot and gear for other players (Wizards and non-Wizards). The closest thing is probably when I carried an extra suit of half-Plate that the Wizard would put on after she ran out of Bladesong for the day.
which mechanics are those, because I am not sure I saw them… the in-step subclass progression? That says nothing about relative power, and yes, I would have preferred getting that
Changing the Warlock to a long rest mechanic, making the Druid Wild more restrictive, Sneak attack once a round, Divine Smite once a round, not rolling all dice on a crit, nerfing twin spell, making Hex and Eldritch Blast Warlock abilities instead of spells, getting rid of Cantrip formulas for the Wizard, making Find Familiar stat blocks instead of real creature stats.
I am sure this is not everything, but it is everything I can think of right now. All of those things were playtested and were unpopular and I believe all of them except the nerf to Twin Spell were reversed in later playtests.
Also there were several spells that were improved, most notably healing and some Cantrips and there were changes that made casters even more powerful than they are now. Those changes further imbalance the game and the response to them was generally positive.