I am not happy that we are being delegated to after thoughts in the vanilla SM codex.
Hyperbole. Space Wolves will get their own full supplement, just like Raven Guard, Iron Hands, and all of those other Chapters got in 8th. When 90% of all Adeptus Astartes units are the same, then it makes sense to consolidate all of the shared units into one book, and use Supplements to cover the unique units.
My immediate question is does this new rule for SW mean we lose the exploding 6's on Assault doctrine that we currently have?
Possibly. We'll have to wait and see what is in store. Considering we literally just got Savage Fury a few months back, I'd be surprised if they were going to take it back away already.
It also pigeon holes your army into being nothing but an assault army. Not ideal.
Not exactly pigeon-holed, but pretty much all of the 8e Space Wolves special rules and stratagems were already pushing us in that direction. Might as well embrace that, and leverage the advantages given.
I also am baffled that you guys seem to like this restriction that you can only take one lord per detachment. Since 9th edition means you essentially will never want to take more than one detachment why would you want to only have one character that can reroll hits? I don't even know what qualifies as a "lieutenant" choice. I don't bother with Primaris HQ choices so does this mean you can't take Iron Priests, Bjorn, Murderfang, Wolf Priests, Arjac etc?
Those of us that are happy with the restriction are, because it is more fluffy. Never really made sense to let folks take multiple commanders (Captains/Wolf Lords) in the same detachment (who's in charge here?). A Wolf Guard Battle Leader is the equivalent to a Lieutenant. So, you're detachment can't have more than one Wolf Lord (makes sense, as you should only have one "boss"), and up to two Battle Leaders (also makes sense, since each Great Company has two Battle Leaders, according to updated/retconned fluff that came in our 8th edition Codex).
You can certainly take Iron Priests, Bjorn, Murderfang, and all of the others you listed, because there is no requirement to take any Wolf Lords or Battle Leaders. You get to pick whatever 3 HQ you want, in a Battalion Detachment, just like always. Only change is that you can no longer bring 3 Smash Lords (at least not without paying CP to add more detachments).
This news should have more people upset. You really want us to have to wait until who knows how long for a supplement while we're given more rules to reflect the SW fluff and lore?
It isn't likely to take very long at all to get our Supplement once the base Space Marine codex drops in October. If I recall correctly, the 6 Supplements that currently exist for the other Space Marine Chapters were dropped in pretty rapid succession.