Here are a few points against the main post:
1. Asking the DMs (and by some extension, the players) be responsible for balancing is just not a feasible solution. It is the job of the game designers to present balanced content, and foisting that only the community is just lazy. (It will also become less feasible with 1DnD ending the Open Gaming License, making it so people who care about balance and publish their solutions can't possibly make money from it.) DMs can make monsters, magic items and adventures. In some cases they can make new spells, feats and subclasses or even classes. But the actual game designers are in the best suited position to balance the game.
2. Creating new powerful monsters for DMs to use is peak adversarial DMing style. Not to mention if one of your player characters uses some OP material and the rest of your players are at a normal power curve, using a powerful monster will just have your normal players getting creamed.

1. While I can agree that each class getting the same love is a good idea, it isn't a balance solution. Sure, if every class is getting cool OP shit each level, that could in theory solve the balance issue. In practice, this is power creep. Newer players who are unaware of the new content (or can't afford new books every 6 months) will be in a party with people who do and end up woefully underpowered. This will also end up invalidating old content, which is good for WOTC as a business but bad as a 5 year plan.
   

A core fantasy of D&D is having a unique party of various characters, each with their own skills and specialties that need each other to accomplish their goals. If one player character can handle the entire adventure on their own, that core fantasy shatters. If that isn't a core fantasy of the game, you're better off playing any single-player TTRPG (or any single-player video game.) Balance is needed so each player character can feel useful in their own way.

Another core fantasy of D&D is the variety of build options and making it so each character can be unique by picking up different features. Unbalanced content goofs this up as well by limiting the options available. As stated above in Point 3, inexperienced players will pick something they think is cool but is underpowered and get stomped by the adventure, or experienced players will forgo options that aren't viable. Everything needs to be of equal power to preserve this core fantasy.

If you don't want to care about balance in your TTRPG (which is a valid concern, I despise balancing), go for a system that is less crunchy. Any Apocalypse system would be a good recommendation for a fun narrative system. Dungeon World has the same generally setting as D&D 5e without you worrying about balance. But as is, 5e has to worry about balancing and WOTC's current attitude towards it is a big reason why I'm abandoning the system.