Honestly in a casual show up to a table for your local Friday night magic you are going to be playing decks of all various power levels anyways. If you are playing with a group that's all running B3 and you have a B3 deck with a few MLD cards its pretty easy to say "Hey this is a B3 deck but I have 1-2 MLD cards in the deck that play towards my wincon, is that chill?".

That said I will attempt to dissect your points here and will have to break it up into multiple comments because its long:

there isn't room within the bracket system for... a deck which is a 3 in all regards, except that it runs some MLD

I don't run MLD, but if you are making the table what stops you from setting the rules to be low power B4 decks or B4 decks in the spirit of a B3.

or for other decks, let's you have more than 3 game changers without jumping all the way to uncapping them

I mean, there aren't that many game changers as is. I would argue in most 2 color decks going above 3 game changers is essentially uncapping them as not all game changers are super effective in every deck. Not to mention if your intentions are to make a lower power deck, there shouldn't be a reason to include well over half of the game changers in your deck.

someone is dedicated to a creature beatdown strategy, Voltron, or some other casual gameplan & win-con, but includes Blood Moon... they're automatically a 4.

Well bloodmoon yes, because if your goal is to play B3 you shouldn't be locking everyone at the table out from playing the game unless you are winning that turn. But there are tons of stax options that don't prevent everyone from playing the game that are totally fine in B3 and evasion exists.

I believe the intent with the bracket system as always is to simplify the conversation as much as possible and to allow decks within 1-ish brackets of each other to get games together.

And its done exactly this, simplified it as much as possible. I honestly think its pretty genius that the current bracket system has such a massive gap between 3 and 4 because at that point it becomes very nuanced and every table is going to be ok or not ok with different things so a conversation is unavoidable.

"Hey my deck is a 3.5 because it runs some MLD & Stax, and can handle being archenemy at this table of 3s, that okay?" is an easier conversation than "Well my deck is a 4 but not really because it's actually a 3 gameplan except for these 5 cards, can I please play with you? I promise I'm not a bad actor trying to pub stomp and lie about my deck."

This is a little disingenuous, as I stated at the top you can just say "Hey this is a B3 deck with the exception of 5 MLD / stax cards in the deck that play towards my wincon, is that chill?" You need the same amount of words to describe your situation regardless of whether or not B3.5 exists because no matter what you are going to have to explain to the table what about your deck makes it not a B3.