Well... I do fully get where the frustration comes from for a lot of the Old daemonhunter players though. Let me show you what GW has done with this army over the past 10 years for people who started out with them in their earliest days:
Inquisition:was there, only ordo malleus was an option though (there was nothing for ordo xenos and ordo hereticus was in codex:witchhunters). You could take henchmen back in those days, so no probs there.
Assassins: were in there. All 4 of them in the book, but you needed an inquisitor to "unlock them"
Grey knights: were in there. Psycannons were pure infantry killers (no S7rending wonders we know today). If you did go full GK infantry, you'd have ZERO anti-tank troops in your army. Hell, there was even a paragraph in the book in army selection that discouraged newer players from taking this type of force (saying you'd definatly need the emperors blessing to pull it off).
Inquisitorial stormtroopers: remember those guys? That was for the people who didn't want to run GKs or wanted to play a radical inquisitor playstyle. Half of your wargear was not allowed if you played with GK infantry, it really was a bad case of "take this to unlock this" and "avoid this to allow this".
Inducted troops: To flesh out the codex a bit, daemonhunter players were allowed to use some entries from imperial guard and space marines. Many took this option, it was all in one book (mind you, they did reference to the "parent codex" to select them). It was your only decent source of getting some much needed anti-tank options really...
Codex Grey knights:
Scratch the inducted troops and inquisitorial stormtroopers. This was also in the days that allies was not a thing... Effectively scrapping over half of my models. I still have 20 painted kasrkin troops standing around and around 40 IG veterans with a leman russ that were just plain unusable.
If you say: "well, you could still use them as IG, so no worries". There is a HUGE difference in playing a large, fully painted 2500p army on a regular basis, to then be told that you now own 2 small ones in a pointscale that your regular opponents never play.
Luckily, you got acces to ordo Xenos and ordo Hereticus inquisition, along with a decent fleshing out of the grey knight section. Still annoyed that there weren't any bikes added though (how much more "knight" can you get than riding your iron steed). Your assasins are still in.
New Codex Grey knights:
Inquisition: GONE, assassins: GONE. All that remains is GK stuff (and that really isn't a lot, I'll do a quick overview in the end).
In order to potentially play an army that you collected over 10 years ago, players now have to buy:
-Codex: Grey knights
-Codex: astra militarum/tempestus (depending on what you want).
-Codex: Space marines
-Dataslates: Assassins (lord knows they'll try to sell you all 4)
So... you can understand why a few older players have their jimmies rustled. I don't think there has been any army so far that has had such dramatic changes to what you could and couldn't field in one source as the old codex: Daemonhunters.
Also, if no new stuff is added, this is the amount of choices GK players will have in each slot:
HQ: 7 (down from 14)
Elites: 4 (down from 9)
Troops: 2 (same as before, but without Thawn as an option), lowest option possible?
Fast attack: 2 (same as before), lowest option possible?
Heavy support: 6 (same as before).
Grey knights already felt "spammy" before they cut out all those option. I'm afraid that won't get any better under these conditions. They'll also feel more powerwhore like for being forced to take so many different sources just to play with your old models.
TLDR: It's been a long essay, but in response to the "just be happy you are getting something" attitude, some GK players really do have a more than good reason to be slightly upset at yet another splitting up of their army.