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My main point was that the level bonus seems to purposefully limit the players to get them to the "numbers" the developers want them at when optimized to ensure the 50/50 treadmill they want is working properly.

Which is also safe to say that they don't plan to release any future content that may disrupt that too. They'll likely release different classes and monsters, with different flavor and fluff, but largely the same mechanics.

Aside from that, it seems to justify nothing. Yes, the wizard is going to get better at balancing (acrobatics), marching (athletics), and even sustaining in the wild (survival), but all these things are a negligible DC and don't require any proficiency to do anything. One could simply take 10, or spend some time rolling the dice in a non-combat situation, to achieve the exact same results.
Ontop of that, anytime they would apply aside from these negligible checks, they're often proficiency gated and therefore useless. What's the point of the barbarian having a +18 to nature if he can't use it to identify something that requires master? He might as well not have the bonus to begin with, because only a druid or ranger with the appropriate proficiency can even roll for the roughly 50/50 chance of passing.

If they wanted characters to be good at their signature skills (or whatever skills they pick, up to expert), then they could add something similar to Operative's specialization (pick a skill group, gets an automatic +1 per level) but allow players to get the same progression of their choice that doesn't go above expert if it's not a class' signature skill.
That way you can have barbarians that can be good at performing, but won't be as good at performing as a bard of the same level, and will only be good at it if they choose to learn to do it, rather than it being oddly instinctive.

Though, if it's done this way, there won't be a need for proficiency gated tasks to begin with.
Especially if the DCs continue to scale as hard as they do, the only ones having an even slight chance of being useful are the ones that chose to invest time and effort into learning it.

Level bonus in this game is both redundant and pointless.
So the premise of it is two things: to allow unskilled characters to do things based on general experience that makes them more versatile (such as a wizard being able to make the acrobatics check to climb a tree, etc.), and to control the bonuses that players can have at any given level to guarantee that their chances when optimized is about a 50%.
The quote saying so is on page 336.

Many tasks use the high-difficulty DC for their level, but circumstances adjust this. For trivial, low, and high DCs, a character with the amount of skill described in the following paragraphs likely has a greater than 50% chance of success, thus allowing more critical successes.

The earlier is evidenced by the fact that the game requires a large number of your beginning ASI to be spread between several stats, ontop of that any stat above 18 is a +1 rather than a +2, thus giving the stats a soft cap and encouraging them to diversify.

Which both of these things are counterproductive to each other, since the DCs are set for optimized characters. If you choose to not do so, you likely can't pass anything above trivial. If the DCs are lowered, it would allow for roleplay heavy characters with lots of fluff to be viable for gameplay and still allow for advancement.

However once again we're back to that the DCs are set for roughly 50% - which there are already systems in place that do exactly that, see the Rogue Trader game from Warhammer where players roll a d100 against their own skill level that's modified by stronger or weaker monsters that lower or raise the percentages, respectively - but this entire system is negligible since it's rare that players (especially in modules) will ever face anything far above or below what's relevant to their levels.
The entire point of it was to take away non-fluff (and in backgrounds for optimizing, some fluff) choices which makes for very boring character creation and RP since an optimized class will only have a choice of two or maybe three backgrounds.

One could still RP despite whatever background they have, however mechanically things remain the same. That a level 20 barbarian and a level 10-ish bard have the same +18 to performance and lores. Meaning that the level bonus alone is six times the bonus given by being "legendary" in the use of the skill.
Now, proficiency gated tasks are there to prevent these bonuses from being abused in exactly this manner, however, if anything relevant (see, mindfog fungus' disable check in doomsday dawn) is proficiency gated, what's the point of having the "other" bonuses to begin with?