The reason you need the gear requirements for tier 3 is because tier 3 is much more brutal. Without your health boost, you'll be down in 1 to 2 hits. Without high defenses, those hits will bring you down even more. The only reason proto armor can survive a hit or two in lower depths is because of "sonic mode", i.e. hits that would otherwise down you instantly leave you at barely any health, much like the Sonic-ring system. The 3* weapons also do drastically less damage than 4* in tier 3. You know the spookat monster? By tier 3 they are very dangerous, as they spew a 3-shot attack, and don't wait around in one spot for you to attack them for more than 5 seconds. Typically they are only standing still for a moment. Gremlins also get a huge attack difference. Thwackers have an instantaneous spin attack that will take out half your health and stun you while knocking you 10 meters away. Menders can also revive downed gremlins and turn invincible for a short duration when low on health. They can also plant a healing rune that heals for ~255-355 health every second an enemy stands on it. If you can't outdamage those numbers then gremlins will be the end of you.

Oh, and all fiends just become more annoying.

Essentially, in order to be in tier 3, you need to be prepared for it. The game tells you to do this through gear, and prevents you from just waltzing through. New players would try to power through that hall of heroes and fail many times instead of level up their gear, then ragequit the game.

In the end, making the hall of heroes a brawl of some sort will not introduce players to the next tier. The right gear will. Making lower level characters access later game parts will only cause them frustration, not accomplishment.

The reason why hotstuff 5* players complain about low difficulty rather than downgrading themselves is because using low level gear isn't skillful. It just takes a long time or ends up being impossible. The problem isn't just that new players would ragequit at the difficulty teir 3 has, but rather that they'd get kicked on sight if they joined a party with people who didn't want to take all day getting through their levels. On top of all that, OOO can potentially make money off of you making gear, so they won't remove the restriction anytime soon. Look at what they did to the oldschool 4* shards bomb.

The idea of having HoH be some kind of challenge is good, since there are a lot of skilled almost 4* people who could easily do T3 and at the other hand, there are also a lot of Vans who should probably go back to T1 to learn how the game works.

Another advantage is that a rank will say something about a players skill too. I have done arcade runs on elite to the core with people with 2* and 3* equipment, and at some T2 runs with Vans I almost started crying because of their fails, not only a lack of skill but also a lack of knowledge ("I know you have BTB and blitz, but that won't help you in a slime danger room").

But again: these are exceptions. With a HoH fight some people would never be able to get to a higher rank. Maybe something like allow the player to choose: you either go grinding for orbs or you beat a challenge far above your level. This way everyone can certainly progress in the missions, but skilled people would be able to not only rely on the luck you need to find orbs, but also on their own skill.