You may tune the difficulty setting (normal/hard/expert/master/etc) for the game until it fits your char and play style.
Unless "Normal" means "multiple levels below you at all times" or "you are a god in a world of dustbunnies", challenge levels don't in any way impact the problems that some people have with a guaranteed-that-monsters-level-with-you system.
The post you quoted included many things that mean that monsters leveling with you become a vastly different experience.
Trying to blast through an act in campaign mode? A lot harder when everything is in your way and as strong as you, meaning you need top-notch gear (minus a bit for Normal challenge) for that level just to attempt it.
Enjoying the experience in Hardcore with a lower skill level? A lot less enjoyable when you're always kept at the same level as your enemies.
Grabbing a few pieces of gear for an alt before embarking? While I know we're getting Loot 2.0 and all, I can't be the only one whose luck with loot is abysmal at best, and being literally unable to try to get a decent item for that character on a regular basis (without a crapton of crafting mats) means that the experience is very diminished.
Just making a character to play through the campaign, already having the gear to be able to be fine as you go? Better not level too far, or that gear becomes useless.
I've always hated the concept of the matched leveling, since like others said, you don't really have a feeling of progression. Sucking at the game and being unable to play at anything above Normal isn't fixed by leveling under this system, it's actually worsened. Back in D2 I couldn't play Nightmare very far because I couldn't get drops to upgrade and died every time I moved 5 feet. Leveling sure did help that a lot, and that's even more true in D3, since you can get new skills and your already-existing ones power up with you as you go...but now if you don't get the gear to have better armor and health than you did a level ago, you're kinda boned since the game's assuming you are
getting that gear, at least occasionally.
Look at it from before the Inferno nerf. A lot of people couldn't do it, and though it was intended to be SUPERHARDENDGAME stuff, people wanted to do it, which led to the nerf and the introduction of MP. However, some people still suck enough that MP0 is too hard for them. On Normal. It's hard to believe, but it's true. With enough perseverance and time, they can maybe find a drop and gain some levels and it'll be the gimpiest stuff ever even to them. But if the mobs level with them? Not anymore.
The concept of a relative level reduction is pretty good to solve the problem for those who wanna have the option beyond just challenge level. Personally, I would like to play with the increased density from Inferno MP1+ on lower difficulties, just to be able to blast more stuff, even if the XP were tuned to actually be the same. If this change to the difficulties adds that, I'd enjoy that a lot...but then being stuck with a ton of mobs that I can never feel truly more powerful over, that'd knock it down a few pegs.
Sure, it'll be hard to know for sure until real details arrive, but Normal would have to be unbearably easy to try and say that it's an option for those who wanna do specific things without the burden of making sure their gear is at the very least medium tier.