You don't "NEED" to upgrade your chassis. Even if you do, it's just one piece out of the 3 pieces for your chassis. You'd be upgrading your motherboard and CPU. You can reuse your wifi, storage, ram, your expansion cards, your display, your speakers, your keyboard, your touchpad, your battery, powersupply, cables...
That being said, you don't need to upgrade from 11th gen to 12th gen. Maybe in a few years when 11th gen isn't cutting it for you anymore you can upgrade to 15th gen.
It's great that they provide a path for upgrading, but the more important thing here is having more recent hardware for someone who wants to buy a framework in 2022.