I think even the most cursory examination of what things were like behind the scenes during the production of the early seasons will amply demonstrate that the story was most certainly Monty Oum's, not M&K's, so long as he lived. I don't believe he had some epic full-length 12+ season vision for RWBY, but the guy was pretty (in)famous for fighting the writers to include more content that he came up with last-minute because he thought it was cool, like Raven's V2 post-credit appearance and the entire character of Neo. He couldn't have done that and won if he wasn't the showrunner.
As I've said before, I don't know the full details about what actually went down between Shane/Sheena and the CRWBY writers after his death (nobody does except them and they don't seem to be in any rush to openly air the old grievances, though maybe that'll change once RT is formally dead and buried). But I wouldn't be surprised if it turns out that the question of whether the animators should remain in the driver's seat for the show or the writers should take over was one of the bigger bones of contention there. Obviously, the writers won out and you can see how that affected the show in the shift to a plot-driven narrative where the characters are pushed along by the broader 'stop Salem before she destroys the world, if you can' plot, the decline of the fight scenes, inordinate focus being placed on things like Bumblebee instead of said fight scenes (despite them traditionally being the highlight of the show under Monty), established Monty-era content like Black Sun or Adam's old characterization getting binned, etc.
And as for the decline of the fanbase, I do think the new management had something to do with it. RT in general has tried to cultivate a 'we're just a gang of buddies having fun together, like you!' image and a parasocial relationship with the fans well before RWBY was a thing, but M&K + Barb Dunkelman and Arryn Zech seem to have kicked it up a notch in the years since Monty's death. They really got into pandering to a select core of especially zealous FNDM types, and the self-insert/wish fulfillment stuff got way more overt in the later seasons as well (I don't mean Jaune either, I mean things like all the political strawmen being put in so they can be beaten up, or the retcons to make Blake the furry princess of a tropical paradise & one half of Bumblebee). Hell they even hired some such people into Rooster Teeth later, like Arryn Troche (shipper who became an animator), and these people proceeded to try to lend support to their favorite ships in their work (Qrow/Clover having homoerotic undertones was the fault of these shippers, that & I imagine they also wanted to spite Vic Mignogna by implying the character he used to voice was gay).
They cultivated a stronger parasocial bond with the crazy tumblrina types who cared about RWBY for 'queer' shipping first & foremost and weren't nearly so interested in its roots as an action show, and basically enabled their bad behavior - these people never called out Wasps being dicks to fan artists or fans in general who weren't into Bumblebee even though they had so much to say about how they hate Trump, think 'SJW' should be a badge of honor and that if you don't approve of 'social justice' they don't want you as a fan, etc. The consequence was that they got to milk more money out of a die-hard FNDM that would fork over the cash as long as they were pandered to, and at the cost of increasingly driving away the non-rabid BB stans & generally making the RWBY fandom space into a much more insular place than it used to be. (Oh, and of course, making the show worse as a result of said pandering. We can see how well that worked out for them in the long run now.)
It's difficult to speculate what RWBY may have been like had Monty lived - according to Shane's letter he was even distancing himself and the show from RT, taking it to his own studio (page 13 or so). But I think it's very, VERY unlikely that the show would have gone down the same path it did, or even gotten close to it; in fact, Shane explicitly named the people that Monty said he wanted to bring with him to continue working on RWBY at his own studio, and M&K were conspicuously absent from that shortlist. Monty may not have been the greatest storyteller and made his share of bad narrative decisions, but the man clearly had creative integrity and a spine born out of a thousand-mile stubborn streak: if he was willing to stand up to his own co-workers & friends that he knew IRL to wedge his characters & story ideas in, I highly doubt that he would have chosen to pander to the likes of the Bee stans (essentially just randos on the Internet, even if they were very loud and angry randos) at the cost of compromising whatever ideas he had like M&K did.