GW has always been about chasing trends and synergy with new products.

In the video gaming space? No.

They've gone through a few different waves. Pre-Relic (so 90s) we have a lot less information on since most of those studios were dead long before game journos even realized they could talk to people but it was very much characterized by "whatever looks like a strategy game". Relic (and the folk who did the underrated Fantasy RTS) kind of had carte blanche in the early 2000s. Yes, there were some clear mandates to incorporate certain armies (largely indicated by Relic outsourcing Soulstorm) but it was a lot closer to what we see with Total Warhammer these days where it was kinda just "make a video game version of 40k".

Then... Dawn of War 3 happened and everything came crashing down. Probably also the MMO nobody but me played and said fantasy RTS that even the fans can't remember the name of. But holy crap did everyone hate Dawn of War 3 to the point I am shocked they didn't remove the number entirely and just call this "Dawn of War: Yo Dog, Primarchs is Back. All them Blood Ravens suddenly feel a strong urge to hang with Magnus But We're Gonna Ignore That"

Which led to an era of slop where anyone who bought a PR person a six pack of cider could get the license. But from what various studios have alluded to in the 2020s, that is mostly gone (outside of mobile slop which has largely slowed down) and it is very much a case where if a game is being made it is because a specific product is being pushed. With Total Warhammer largely being the last holdout since even GW realizes nobody cares about Age of Sigmar (it amuses me that probably one of the biggest supporters of AoS online is Louise Sugden who can't go a single video without all but daring GW to come at her and get her to explain why she left).

So if the fricking Mechanicus is an army: There is a reason. And it very well might be the reason for the game itself.

Tau I think kind of need to be a DLC/Expansion faction.

The problem is that a LOT of the people who "grew up with" 40k still think they are just space commies with Gundams. GW, terrified of Politics in their game about space fascists, played their trap card by reminding everyone that the uppermost caste (aetherials?) of the Tau were always kind of... monsters who rip the free will out of their subjects and that is WHY the Tau seem so internally stable compared to every other faction. Which, in turn, made everyone (self included) make even more jokes about them being space commies.

So launching with that is a great way to get the wrath of a million chuds unleashed because their game about space fascists is now woke.

But yeah. As someone who has always loved the Guard, the Tau are kind of that cranked up to 11. With Gundams.