If same-level sidekicks are actually outshining player characters, I would be shocked. They may have one or two nice features per specialization, but they are dramatically less powerful than player character classes. I'm running a lot of them in my games, and they're mostly good for dragging away unconscious characters and performing Medicine checks on them or distracting the enemy while the rogues position themselves for Sneak Attack.
They tested a Level 1-20 option, but published a Level 1-5 box set. Methinks they had something else in mind, which they did eventually publish.
Van Richten's Guide was announced precisely three weeks before the release of Candlekeep, Tasha's about three weeks prior to the release of Icewind Dale, and Icewind Dale was announced a full month before Theros hit stores, so that would seem to align with that pattern-wise.
Running a game where the PCs are all sidekicks would be a great way to reduce power levels of the game.
I've sometimes wondered if it would be worthwhile to go through the classes, grab some of the class or archetype features, and then have people play a sidekick class with one of those features. Like, you play a Warrior, but you can choose a couple of Fighting Styles, or Rage or Smite. And that would be it. You wouldn't get any other powers from the class, but you'd have just enough to differentiate you from every other Warrior out there.
I've had similar thoughts. Reward the some of the more powerful abilities as boons or cyphers or magic items. Could work.
Sidekicks can easily be better than 1st-3rd level characters (if I've been reading and applying the rules correctly), after that, they start to lag.
It depends on the specialization. The protector warrior is fantastic at those levels, no question. But no one is going to prefer a healer sidekick to an actual cleric, even if all the cleric does is heal.
There's sidekicks up to 16 or 17th level on dndbeyond.com
The other levels where included in the free-to-essentials-owners modules on D&D beyond.
