I mean, Mascot Hazel is one of the best healers in the game and IS the best fire team healer. F.Kluri is a top 10 unit. Still. Roozid is arguably the best CR pusher in the game and offers a ton of utility. Lorina is one of if not the strongest single damage nukers in the game because of her flexibility. Axe God is an absolute monster. Taranor Guard and Alexa are both nearly equivalent W11 killers to their 5* counterparts. Carmainrose is still stupid strong. And that's just 3* which everyone has, if we open it up to 4* you have Karin, Angelica, Achates, Cidd, and on and on.
What event has required 5*? Every event is super easy, and it's to the point where long time players can clear Hell with 2+ fodder.
Arena will always be shit. It's gacha PVP. It's never good. But if you don't invest your life and life savings into PVP you can succeed in PVE quite easily by playing smart and using units that work well together.
I'm pretty sure Tama >>> Mascot Hazel, possibly Achates too. I'm not sure if there are any more fire healers. I agree with you in the Mascot Hazel is a good healer, but pretty much all healers are useful if you invest in them.
F Kluri is a good unit, but again she was better before (since she got nerfed). And she was the only one to get nerfed, while other popular units like Ruele, Diene, Assassin Cartuja were untouched, which doesn't contradict SG favoring 5*.
No disagreements on Roozid being good for PVP, but I did mention that that is one niche where 3* are still good, mostly due to easy access to max speed imprints.
Lorina is a nice DPS, she also got nerfed (before global launch i think) and has no utility. And all the new units are starting to beat her at her niche of DPS (Luna, Cermia, Bellona, Sol, etc)
No Axe God, so i can't comment on how good/bad he is. He is a ML though, so he's not exactly easy to get.
On 3* being good for W11, its true, but at least half of the success can be attributed to how good Daydream Joker (& % health damage in general) is in PVE.
On event difficulty, you are definitely overestimating how much the average person plays. Half our guild (Epic Era) isn't even player rank 60 yet, and most definitely cannot run Hell with 2 fodders, I doubt they can even run Hell. Even though i play a fair bit, I only recently started doing Hell.
I've never said that 3* are bad, but SG wants them to be useful in certain quests only. For me at least, I have a core team of characters (mostly nat 4-5*), 3* are pretty much raised for a specific niche (e.g T Guard for W11) and that single niche only. If you get lucky and have the right units, you can probably do all 4 hunts with maybe 6-7 high tier characters, while if you use 3*s only, you probably need 12 different ones, and use Kluri in most of them.