If we accept the idea that different tone, writing style, worldview, aesthetics etc makes for a different parallel universe, then Star Trek's 'prime' timeline is already a multiverse. The many worlds theory of Star Trek as I see it:
1. The TOSverse - ST:TOS, ST:TAS - primary coloured, sharp angled sets, episodic, vague and changeable on background details, we are a small fleet of identical ships on a Wagon Train to the Stars type exploratory mission. Sixties fashions, sexism and sledgehammer metaphors abound. The late 20th Century of this universe is a spacefaring and dangerous place, wracked by war. Most aliens look human but some have some basic additions like body paint and antenna.
2. The Motionless Universe - ST:TMP - we are now in a grey, brown and beige parallel world where the Enterprise is different, we are Earth based and the UFP and Starfleet have solidified into a kind of future UN of the Stars, the tone is completely different, slow paced, long lingering reaction shots, the Klingons are totally different, and Spock is like a brand new character.
3. The Marooniverse - ST:II-VI - A lot has changed again - beige and grey are out and bright red and the famous 'off' colours are in with a complex rank and division system we'll never see before or since. Now we are Horatio Hornblower riding a submarine in space. Anachronistic retro spaceship universe, with 'right full rudder' and ship's cooks, labels on everything, and the truest military Starfleet has ever been. The Klingons have changed once more. The Vulcans have immortal souls like for realz. Other aliens also look a lot more... alien.
4. The 24th Centuriverse - TNG, DS9, VOY, the TNG movies - We are in a shiny new future with a whole new look, skintight catsuit costumes and a pretentious utopian vision which slowly fades from the writing as this universe progresses. We don't have countries, money, flags, or a sense of humour, and our society has stoically refused to be changed by the revolutionary invention of the replicator. Writing is quite stilted and precise, and there is little conflict between characters. The 20th Century of this universe more closely resembles ours, but the 21st is a time of great destruction and governmental collapse, leading to First Contact and humanity reaching to the stars. If there's one thing the humans of this universe are sure of, it's that they've left all the bad stuff like war and disease behind them. Except they seem to constantly start wars and contract weird diseases.
5. The Discoverse - DSC - This universe is blue. Very blue. Humans have some pretty high and mighty ideas but don't seem to actually live up to them all that much. Space is dangerous and violent, and yet contains only a handful of people. The tone now is 'EPIC AF' and the writing grandiose and expository. Big sets, big plots, and a different type of character than the other universes. Oh, and the Klingons have changed again.
As for ENT, for all the stick it gets for being the red headed stepchild, fits broadly well as a prequel to any of the above universes, and indeed the Kelvin timeline, but probably sits least well in the TOSverse. It is best suited perhaps in the 24th Centuriverse, which it seems the most direct prequel to.
So exactly like TNG, then, really. That show too took what it liked from the original, threw in some stuff from its contemporary movie series which didn't actually fit in all that well, redesigned other stuff indiscriminately, and made up some aspects and features from whole cloth, some that should have changed the whole setting but didn't, and others that don't really fit with what went before but will be broadly ignored.
Cart before the horse. Trek 2009 could have just presented a different version of TOS without explanation, but knowing what fans are like, JJ decided to explicitly explain it in the movie to try to keep the hoards at bay. He didn't cause the issue, he was trying to deal with it.