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Secondly, it would act as a disincentive to give merit away knowing that it's just going to decay way making your overall merit lower.
Only airdropped merit would decay under theymos' proposal above, not earned merit.
Yeah that's what I am telling that this could do more bad rather than good since either way the ones who will be abusing the merit system won't be affected by the changes at all the only ones who will be are either inactive accounts or accounts who won't be buying any merits.
I don't think that's true.
When theymos introduced the "1 merit for Junior Member" (and therefore a signature) requirement, there was a noticeable and measurable reduction in spam over the following weeks:
The new rule (1 Merit for Jr. Member) is already reducing spam. This is despite all the merit selling and awarding to alt accounts that went on. There would be presumably be less merit selling and alt accounts now, since a lot of accounts involved in this will have used up much of their airdropped sMerits, without earning any more.