First of all, they confirmed Loki 2 was one of the few (if not only) MCU shows that didn't undergo reshoots, so what's on the screen is what was initially envisioned. Plus they wrapped long before the strike so I'm pretty sure the lack of reshoots was not
because of the strike but rather because they genuinely were satisfied with the product.
Second, the
only thing I can figure as far as how important the final episode made Kang is..
maybe because Immortus (I really don't wanna call him "He Who Remains" ffs) dropped that little line of "My variants are already out there..." and because there was some heavy talk in this episode about how Kang variants will be waging all-out war and destruction if Loki allows the multiple timelines to continue to exist (which Loki then did). So that "dealmaker" might be under the impression that if Kang doesn't continue to be The Bad of the rest of the arc, it'd be an extremely awkward thread to abandon since we as an audience know what they're talking about in Loki is what's planned for the future of the whole franchise (which is arguably the only reason it even holds any weight in Loki).
Granted, with how fast and loose they've played the "variants from different universes" angle, I could easily see a pivot where they decide one (or several) of Kang's variants turns out to be Dr. Doom and then they go all-in on Fantastic 4. Or they could decide some super sci-fi bullshit happened and all the Kang variants joined and morphed into a single, massive, planet-eating being and then they
still go all-in on Fantastic 4. Kang in the comics has ties to the Richards family anyway! And before Kang was announced as the Next Biggie I was genuinely expecting Doom and Galactus to be this saga's Loki and Thanos, it just kinda made sense and it possibly still could.
It's sorta funny: they've written themselves into a situation where the "multiple universes" angle makes all the stakes and character sacrifices and plotlines kinda bullshit, but at the same time it means the disastrous casting issue they're facing can just as easily be bullshitted away with "because variants."