Firstly, leveling from 1-20 over a single campaign would just be poor design. Characters don't go from complete rookie to demigod superhuman in a few weeks. The world needs to make more sense than that, and D&D makes more sense than that. It would just be really bad pacing. From a mechanical standpoint the levels wouldn't be worth much if you just speed through them.
Secondly, I'm not buying the "everyone is legendary but the tadpole weakened them" narrative. There is absolutely no point for the Mind Flayers to mind control these characters if they are also weakened to level 1. You said it yourself, they were targeted because they were powerful. Also, the True Souls did not get weakened. This is quite simply a complete disconnect between writing and game mechanics. They should support rather than undermine each other. I find it incredibly off-putting that Gale is boasting about his powers when he's actually casting Magic Missile at Goblins.
Not to mention Zevlor refers to Wyll as a "Legendary Monster Hunter" yet he is Level 1 when we meet him.
