Then I think you are extremely naive. In most gacha games, the new content usually revolves around re-incentivising spending through new systems - e.g. 'a new currency that allows you to buy SUPER LEGENDARY GEMS that make the old legendary gems redundant'. If you want to stay relevant, you'll need to acquire these new super legendary gems, you can earn one a month through the SUPER BOSS but luckily Blizzard has another way you can acquire them...
I think given the insane predatory practices adopted in Diablo Immortal, could you honestly say you'd be suprised if they didn't pull this as their next content update?