Ex Raids are not dead :(
In the upcoming update you can share an ex pass with 1 friend.
So who will be the next Ex boss?
Concerning who our next EX boss will be, nobody knows until we know (aka Niantic tells us). Personally, I think Arceus would be a good pick (though they'd have to change the rule about not being able to transfer mythical if this is the case), as it has powerful base stats, can take on any typing in the game, looks cool, and is a well-known Mon. I know some have suggested Deoxys, but I hope it isn't the one selected as the next EX boss. Deoxys is another novelty Mon with stupidly high attack and pathetically terrible bulk. Sure it has 4 different forms, but I don't see people being hyped over doing Deoxys raids in the long term, as it's usefulness will be nonexistent. Niantic would need something powerful and interesting enough to keep people caring about doing EX raids. Arceus fits the bill pretty nicely, I think.
The ghost pokemon had been released, and maybe a very small number of others. And in any case, there is absolutely no basis for your argument that they can't release Arceus until gen 4 is released. What if they decide to use Arceus to launch gen 4? He could either be the first gen 4 mon, or he could be released contemporaneously with a few other gen 4 mons.
They won't stop. But because Mewto is a normal Raid Boss, no one will care about Ex-Raids anymore... this is why you will be able to share invitations. This way at least someone will participate... IF this feature really will be implemented... its just a rumor...
Edit: But of course, this is all just speculation ;)
And by the time Mewtwo month is over, we may very well see the launch of Gen 4 in October, paving the way for a Gen 4 EX raid boss. Besides, what Mon between Gens 1-3 would they even pick as a new EX boss? The only unreleased legendary/mythical would be Deoxys, whose EX raid hype would die within a month. They could reestablish Mewtwo as the EX raid boss, but after a solid month of Mewtwo as a regular tier 5 raid boss, the excitement factor of getting an EX pass for Mewtwo would be pretty much nonexistent.
Did some new piece of news or update come out that I missed or is this in relation to the accept/deny bits of code datamined from the game master?
Either way, my money's on Deoxys. I think long term Arceus will become the EX boss at some point, but given we've yet to hear anything official about Gen 4 despite their blatant acknowledgement of it months ago via promo art I think it's a bit early to call Arceus as the next EX boss.