PDS tend to make DLC/expansions as long as a game is doing well, so it would be very hard for them to judge how big a 'season pass' would be pre-release of a game (although it's fairly safe that HoI4 would do well). There'd also be a 'sticker shock' issue if they expect to produce as many expansions as for CK2, say, which has done very well so has had so many expansions that even if reasonably discounted, would (appropriately) cost more than the game itself.