That's basically the argument Boeing is making. However, there is a counter argument that the FAA could make (though I don't know if they are): in order to limit the risk exposure now that this issue has been discovered, the same mitigation should not be allowed on any new variants not already in service, even though it is allowed for the ones already in service, because taking variants out of service is a much bigger deal than not allowing new ones into service.
But if that was the argument, then that would also mean that we shouldn’t allow any newly built 8s and 9s to enter service, and I don’t see that happening.
Yes, that's a fair point, although doing that would require some sort of modification of the existing certification of 8s and 9s, and I'm not sure how that would work. If those certifications were simply revoked, all 8s and 9s already in service would be grounded until the issue was fixed. But if they are simply left alone, new 8s and 9s can come into service since those variants are certified. An Airworthiness Directive, which is what is currently issued by the FAA for 8s and 9s, by itself doesn't prevent new units from coming into service.
If the plane’s issue is the exact same issue and the mitigation is the same mitigation that’s already been approved by the FAA as effective, yes, and this is currently how things work. See the MEL. If mitigation X is approved for situation Y, then it is approved for all situations of Y (within the same context). You don’t have to go to the FAA to get a new approval to take off every time Y happens, you just do X and you’re approved.