Oh god, please no!
Well.
But in any way:
Head-Canon. Itās when you realise that there wonāt be a Californian Inquisition even if you dissent with Paramountās/CBSās/whoeverās interpretation on whatās Canon or not and decide to just make your own decisions. 
I just really donāt want half-mile long starships showing up in Disco - the 500 m + models are bad enough 
Just because those two have finally merged does really not mean that the Kelvin Timeline will be added to DIS.
How? the only thing I can think of is when Disco time jumped, they jumped to Kelvinās future, not the prime timeline
I really doubt that they would do that!
Only those who really hate DIS would want that, because it would give them just more reasons to hate DIS.
Because hate is what Star Trek is all about! 
To see the fandom acting out like they do, youād be forgiven for thinking that.
Thatās only a smal (but extremely loud) minority of the fandom. They believe that they, and only they, are the gatekeepers of Star Trek canon. 
Yeah, it was that kind of thing that put me off Star Wars. Iām too invested in Trek to abandon this fandom though 
Iām not saying they would or should, but if they did, that seems to be the least convoluted way to make it happen.
Honestrly what I suspect it means is āIād much rather we stop producing films in an alternative timeline and go back to making films taking place in the timelineā ala the first 9 trek movies
I agree with that as the best way forward @BrianDavion.
There is, however, no reason that the Kelvin timeline canāt be acknowledged as an alternate timeline caused by the time travel, that happens in parallel to the prime timeline. The mirror universe is already a big part of the prime timeline after all. Separate, but part of the same universe.
Star Trek is from an era of television when special effects were expensive, so they were rare. The stories had to involve clever sets, make up, and costuming to get the alien-ness expressed to the audience. The stories had to be character driven, not action or effect driven though there was certainly action.
In the last 20 years, effects have become cheap. Audiences flock to movies and shows that are weak on characters and story for cartoon effects from ship combat to aliens. Dedazzle gets butts in seats and clicks on sites. J.J. Abrams and Kevin Feige have made more money for Star Trek, Star Wars, and Marvel than Gene Roddenberry, George Lucas, and Stan Lee did in just 10 years.
They will not go back to character driven, thoughtful, self reflective, speculative science fiction in Star Trek until this fad passes or this bubble bursts-- and it hasnāt passed in 20 years. CGI superheroes and space battles have been the bread and butter of Hollywood for two decades, itās the longest running fad in entertainment.
Get used to it.
I disagree.
I think that intelligent storytelling is moving away from the movies to the TV screen, because you have much more time to tell your story in a TV show than a movie. Game of Thrones would not have worked as a movie or a series of movies.
PIC for example is not a show driven by special effects, it is a drama show where the characters and their development are important. DIS however is an action show but even there the special effects are not the element that keeps the story running. They could have made both shows 20 or 30 years ago and they would have worked quite fine.
The huge problem is that since Star Wars and Alien intelligent SciFi movies have become extremely rare. SciFi had become the background for either special effects driven action or horror movies.
This also means that it is highly unlikely there will be a follow on to the Kelvin movies any time soon because of how badly Beyond did at the box office.
Major studios will have no interest in directly competing in a genre with something as dominant as Star Wars with a product that has already failed for them.
Hopefully this means that Trek can firmly sit back to being a small screen product where it belongs - a format that does allow for something a bit more considered and well written, even if it continues to lean heavily on the glitz and glam of battle effects as you mention.
I expect itās much like DCās multiverseā¦ all the versions of Batman exist somewhere.
All the Treks exist, including Treks weād rather werenāt taken. They arenāt all part of the same timeline or quantum realityā¦ but they exist.
The problem with the JJ films is they are big box bang up explosion filled expanses lacking plot or connection to what made Star Trek great! The last movie was closer to the model that has carried ST through the years.