I'm not quite convinced by Arch's logic in this case, a lot of it comes off as just complaining about any sort of change. There are certainly issues, in some cases major issues with some of thier new stuff, such as some of the depictions of Ultramar in the new comic, or the precise way the new primaris marines have been rolled out (such as the fact that, 3 years in, they still have next to no customization for thier HQs). But overall 40k is doing alright these days.
Arch's main concern of warhammer "going mainstream" seems...off, I think. Warhammer has never been a giant of the sci-fi world, but it's always been mainstream SF, with tons of books and videos games to it's name, many of them extremely well regarded and very well known. Also, many of them do exactly what Arch and some of the more grognardy fans complain about, namely toning down the more over the top grimdark elements of the franchise.
As for his complaints about the "mythical wider audience", I think he's again slightly off base. I can't think of a lot of franchises that have sold out in order to target just a "wider" audience, rather, as his example of STD shows, they've switched gears to try and target a different audience. The most obvious (to me) examples being the many RTS games that have retooled themselves to try and become the next starcraft. Thus far, warhammer doesn't appear to be going that route, though it's hard to tell what they are doing.
Right now, they mostly just seem to be wanting to get an animated multimedia division off the ground ASAP, hence why they've been snapping up nearly all the high profile creators that have already built an audience or show promise, rather than taking a more conventional route. What they'll do from there is up in the air, but I don't think it's going to ruin everything, most likely it'll just shift the overall tone to something more in line with a slightly more grimdark Ciaphas Cain, rather than the "classic" era's tone of "it strains credulity to believe the imperium hasn't already collapsed under it's own weight"....which I think I can live with.