If a fork have five teeth when it's in new, it's perfect.
If two teeth of a fork break, it's broken...but still usable.
If two more break...it's impossible to use for some foods, but still have its use.
Break the last tooth and you end up with useless crap that can't even work as a lever because forks are made from too soft of a material.
See that?! There are multiple stages of "breaking" stuff. Current system is not "perfect" mechanically, because it doesn't let you get that feel that there was a point to you growing stronger. And if you unsync it you cannot get any drops (experience is not relevant here) except from bosses. However, it's still usable and works while minimizing the negatives.
Your system would completely break it. The difference between high level and low level players would be massive. And that's unavoidable. Even if you balance the damage output of high levels to compare to low levels...what point do you compare?! I mean, at lvl18 playing a bard is laughably easy. At lvl70 playing a bard is a very demanding task. That means that a lvl18 player (Sastasha lvl cap) will use their available skills to the utmost, meaning that even if they are average players they will bring damage near the classes maximum output. But a player that's lvl70 synced to 18 in your way?! Not really. Most will be below or around the middle of the potential. That means that at lvl70, you are WORSE than at lvl18.
Go down with the balance to the other end of the spectrum, and nearly every lvl70 WILL be better. And that invalidates the whole process of balancing.
Go down the middle and...some players will be far weaker and some far stronger. Not to mention, the other players will be unable to get a good role model. They will find themselves copying poor gameplay or thinking that a good player is bad because relatively he's at 80% not 95%, unlike the lvl18. So the lvl18 when getting to level 70 will avoid that rotation...and is more likely to end up way lower than at 80%, rather than higher.
What I agree with is that early levels are a pain because most classes lack core FUNCTIONS to themselves. Like AoE, being the biggest culprit. Even if one, every class should have access to AoE from the very first dungeon. Every class should have access to one ranged attack from very first dungeon. And every class should have access to their core mechanics at lvl15. Stormblood made low level content significantly more of a pain because they actually took away from early levels to have what to give at the new ones. That's a design flaw that they absolutely SHOULD fix. And only that.