I don't see why that's a problem. It's not like other Paradox games where you're just playing through history. Hearts of Iron IV is about World War II, and when World War II is over, the story is over.
Now, if what you really want is a grand strategy set in the WW2 period that plays more like Paradox's other games, then that's something I can get on board with.
We are playing a War Game. It isnt a lackadaisical trip through the 1st half of the 20th century. It is a titanic struggle for the very soul of humanity.
There is little point in prosecuting wars until global peace because the game is over as soon as you do.
I see two main reasons why victory conditions should be implemented eventually (well 3, but the third just opens up a window for more content).
I see victory conditions adding to immersion as well a improving gameplay, particularly for MP.
Also, I can only speak to a historical playing of the game.
One major problem with HOI4, especially in MP, is the game often plays out UK/USA/FRA/USSR v GER/ITA/JAP (+2 minors).
The actual war was a lot closer to:
UK/FRA/USA v GER/ITA
GER/ITA v USSR
JAP v USA/UK
JAP v China
With the note that UK/USA/FRA all wanted China and USSR to win, but only just, they wanted the communists and fascists to bleed each other to a nub.
That dynamic, where the Western Allies knowingly only fund USSR and China enough to avoid their own destruction. This would also likely require some rebalancing, likely making fascists a little stronger thus requiring LL, but that is another discussion.
Further, this would help model the race to "liberate" Europe. The UK wanted landings in the Balkans to stave off the Red Advance west--USA was more interested in not sacrificing more Americans than needed (among other factors).
This could be achieved, generally, by:
Western Allies: USA/FRA/UK/ Commonwealth
Axis:
Europe: GER/ITA/HUN
Co-Prosperity Sphere: JAP/MAN
Commintern:
USSR
PRC
China
(DOD minor could sub for Hungary, China warlord could sub for China, Finland, Spain and other belligerents as they get focus trees)
To Win:
Western Allies: Fascists must capitulate; must have more victory points than Commintern
Axis: Must not have capitulated by VE and VJ days respectively
Commintern: Fascists must capitulate; must have more victory points than Western Allies
China sub victory: Japan off Chinese Mainland; More victory points than PRC by the end of hostilities
PRC sub victory: Japan off Chinese Mainland; More victory points than China by the end of hostilities
victory points would be something pegged to CIC/MIC/Population under a given ideology and war participation etc. I'd leave it to my betters to devise the formula.
The immersion would come from fearing the communists almost as much as he fascists, it'd come from that moment when you know youve won WW2 and so begins the phase of securing the new world order.
The gameplay seems obvious to me. It would change the dynamic between the ideologies adding a new strategic level of thought to the game.
It also opens the Door to the Cold War. One could make the UN in their game and set up a NWO....conveniently Paradox could make a perfect segway into Victoria 3 : The Cold War
In SP I can do all this now, but it's all in my head. And I suppose I should rephrase, do I do this now.
But it would be a nice function to have in game for MP. As at least when I was playing MP it was EVERYONE GANG UP AND ORGANIZE TOGETHER TO BEAT THE NAZIS! (which seems like a good argument as to why a Germany may exploit to keep up)