Yeah, obviously they’d still have bugs; everything does, but the two original games are so broken they don’t even launch without installing 20+ mods and it’s clear playing either that a near full to full rewrite is the way to go. They could do that and also just replace the original but that would mean taking away peoples access to an existing mod ecosystem, so releasing it separately makes the most sense

And of course they could give existing owners a free copy of the new version but if they’re redoing the whole game then its equivalent to a triple A game studio making and releasing a large game for free which would just remove any incentive to actually do the rewrite