Does this mean that sidemount is now mainstream and DIR compliant?
Not mainstream but "DIR" compliant after the necessary "DIR" training and proper (according to "DIR") utilization.
None of the "DIR" agencies I know competes on who did it first, this goes against all purpose. In the worst case there is a competention on who does safer, and only limited to specific cases.
It should be as I’ve used sidemount with a longhose and necklaced shorthose for years. Can barely tell the difference except for breathing both sides down and it’s so much nicer than backmount. I digress.
Good for you, I guess? What is the rational behind requesting GUE changing their standards to fit your perspective? Having GUE agree with you and approve your style of diving? I know of people that dive for decades without being certified, even beyond recreational limits. Should also GUE endorse diving without training and certification because some people have done it and believe that it's ok? It's a bit hard to follow the purpose of this post.
Seriously there are a bunch of other scuba organizations out there that already can support anybody to switch to sidemount with minimal effort. Nobody is pressing anybody to get in GUE and remain a GUE diver. GUE is not even popular enough, such as PADI, to be a monopoly. Every GUE diver (with few very very lucky exceptions) found GUE after a lot of search, joined GUE after a big investment in money/time/effort and choose to continue be a member of GUE every single day. Every GUE diver choses to not dive with a sidemount before they become experienced enough for GUE standards. So... if you are asking why GUE divers choose to not dive with sidemount in open water, the simple response is because it fits better to their style of diving, the same way you prefer sidemount in open water instead of backmount.
Again, to stress, this isn’t a dig at GUE. It’s to point out that diving wasn’t invented by DIR which evolved out of the specific circumstances of Florida et al caves.
Nobody in GUE believes that diving was invented by "DIR". Not sure where you got this impression. Even back in the time they chose "Doing It Right" over "Doing It First".
Also "DIR" indeed evolved out of specific circumstances of FLorida caves, but it doesn't seem that it cannot be applied universaly with few exceptions that I am anaware of (possibly due to lack of experience). Also the Internet was developed from USDD, but here we are using it to argue about minor details on a recreational sport. It looks like the same techniques that could be used for fast encrypted communication for military operations, could be used also for this discussion. Similarly it looks like that some rules for safe cave diving in High Springs Florida, could be used for safe diving in open-water, caves, shipwrecks, etc in the other side of the planet.
-------
I haven't see anybody here claiming that diving your way is something you should stop doing and convert to GUE. All the GUE members in the posts I read simply say that they prefer diving the GUE way and that they consider it a better fit for their diving style.