Why not… instead of ending at series 77. Make one more. Call it “series finale”. Then just let it loop.
Instead of vehicle rewards or anything just reward Cr. So if you choose not to participate you don’t miss anything serious.
This way the achievements aren’t forever dead and new players can still try and get them.
Or…
Remove the time restrictions from previous series and let them just become another part of the game. Allowing everyone the chance to fully complete every series and make it another grind.