There is absolutely no problem with this Kind of Inflation... Although it is not really Inflation but growth in monetary supply. As long as the growth of the platform is stronger than the increase of the monetary supply, this type of inflation is healthy. Actually, it is necessary.

Regarding your proposal on downvotes: I am not sure that it would Work, but it could as the downvotes are an etfective cure for bad content paid. But people could still upvote their own content last minute and would be rewarded.