Let me use deduction to ascertain why I believe Sargeras is in 7.4:
Firstly, suppose Sargeras isn't in patch 7.4, what will this mean currently:
The last boss in the 7.3 raid is Argus the Unmaker
, unless this is a "placeholder" name for a significant lore character, then having a `random' unknown boss be the last boss of an expansion does not make sense.
At the beginning, Ion Hazzikostas stated: "We are going to Argus to defeat the Burning Legion. To do that, we need to defeat their leader."
is not their leader - Sargeras is. Blizzard has never made the end boss of an expansion an unknown person.
That means, it is either the case
1) The last boss in patch 7.3 is a placeholder name for a significant lore character.
2) There will be a secret boss after
that is revealed after he is killed.
Patch 7.3.5, a insignificant patch, would never introduce another raid with Sargeras in. Therefore, assuming
is not a placeholder name, then I doubt there will be a secret boss that will reveal themselves after he is slain.
That means, since there is always a significant lore heavy character at the end of every expansion, and, the fact that Ion said we are going to defeat the "leader of the burning leader", leads me to believe that, Sargeras will be in Patch 7.4