I think probably at times new tags will get maxed out as people chase trends but many old, relevant tags still get used and for curatirs filtering by tags the hassle for these things changes and maybe some curators specifically track trending tags but, well, mostly I don't think it is a problem.
But for some time, since at least september, there has been discussions about moderation systems based on renting names. This will be eventually implemented in some form and I think maybe subcategories of flagging, like specifically flagging a tag on a post and downranking it in the tag so it falls lower on the tag trending and hot view, for example. With antispam features to limit use for mischief, of course.
The new affiliated sites coming up will also include moderation systems. Like, imagine a curator name system with subject focus... It just requires a different viewer.