I believe you are incorrect.
How dare you question a squirrel's assumptions?!?!
The story we are given in the Loki series is that there is no clear "main" reality. Kang mucked about, and discovered a bunch of different versions of himself in various realities - but then some of them tried to take over other realities, and they went to war with each other.
To be honest, I don't remember the whole plot of the Loki show. It's been a while since I saw it. That being said, was it stated there was no clear main reality? I could have sworn the images they showed with the sacred timeline appeared as one timeline with other timelines branching off from the main timeline, like this.
Eventually one of them killed off the rest - and that's He Who Remains. The Sacred Timeline he sets up has no greater claim to worthiness than that it is created by the survivor of that conflict - history was literally written by the winner! And, the implication I got is that it isn't his original timeline - it is an edited thing built so the TVA can maintain it and keep other Kangs from showing up...
I'm not sure where the claim to the sacred timeline or any other being worthy comes from. I don't think i've ever made that claim. If my posts has somehow made it appear that I believe the main timeline has a claim to greater worthiness, it was not on purpose.
What I have attempted to communicate is that the other timelines, being offshoots of the "sacred" timeline, are destine to be destroyed along with the sacred timeline
But remember, the only threat that comes out of allowing multiple realities is still just Kang! It isn't like he's protecting reality from anything other than himself. It is basically a protection racket.
The end of Doctor Strange 2 Charlize Theron presents the convergence events, which if they are anything like the convergence events in Jonathan Hickman's (2015), have realities crashing into each other and causing the destruction of the realities that crash into each other. The comics has the Beyonders causing the destruction of the multiverse. The MCU has yet to reveal the cause of the convergence events, or maybe they did, and I just don't remember.
In any case, in the comics the convergence events are stopped by destroying that realities earth. Earth seems to be the Florida of the universe and the cause of everyone's problems. I'm going to assume that the MCU most likely won't use the Beyonders as the main villains, and destroying each reality's earth as the way to stop the convergence. I'm thinking they are probably using the destruction of that universe as the way to stop the convergence events.
So basically, Kang is a threat to the multiverse and so are the convergence events. But that's if they have some level of similarity with the comics.