Current seasonal gear should be affordable and easy to level up, with minimum restraints.
Older gear should cost more resources to level up, and require a specific currency that enables players to push their gear level caps up higher.
This is fair for everyone.
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This doesn't really address the reasons they are sunsetting weapons though. They will still have to nerf PVE weapons more often because they affect the sandbox forever, and the weapon designers will still be shackled with having to build weapons around that fact that they will last forever and affect content forever. So they won't be able to get more creative, powerful options out there for us to try. Everything will be the same boring things we have now, because everything has to be kept that way since it never goes away.