The d20 rules break down at higher levels, as shown by the epic rules and Immortal's Handbook. Mythic tiers are independent of level because most parties generally do not get that far, but fails to address the break down in favor of accelerating it. This is not a bad thing. Even without reaching epic or mythic level characters will become superheroes beyond level six or so. That does not even begin to address idiosyncrasies like the "Christmas tree effect" or the "fighters can't have nice things" mentality. Essays such as "Calibrating Your Expectations" and "Gandalf was Only a Fifth Level Magic-User" explain that levels beyond fifth or so are supernatural compared to the real world and that fictional characters (uninfluenced by D&D) generally are not written with levels in mind. AD&D even had NPCs all stuck at zeroth level to indicate their lack of combat prowess. Pathfinder ignores this reasoning and decides that the cut off point between normal people and player characters is tenth level (cf. NPC Codex), even though nobody on our Earth could be higher than fifth if we operated by d20 rules.
The problem in this case is one of feel. Mythic is advertised as making campaign more like mythology, but the standard campaign is already emulating mythology. The party is saving the world, fighting demon emperors, blowing up death stars, starting galactic wars, exterminating space zombie apocalypses, and so forth without the assistance of the mythic rules. Adding the mythic rules doesn't make anything feel more mythic than before. Aside from things like granting spells to your worshipers, mythic powers are generally unimpressive numerical boosts. Mythic monsters are more powerful versions of monsters that are already mythological. Beyond equating mythic ranks/tiers with divine ranks and thereby stating that mythic characters and monsters are literally gods whereas Joe Shmoe adventurer is merely a mythic hero, I cannot think of a way to make mythic feel inherently mythic or more mythic than usual without switching to a level-less system.
How does a GM make mythic rules feel different from non-mythic rules?