Most of these puzzles aren't close-worthy as such: we have a formation-of-numbers tag with over 150 questions nearly all of the form "make
<number(s)> using only
<operations>". Some of these are fun and challenging, while others are pretty prosaic, but on the whole I think the community does a pretty good job of sorting out the interesting from the uninteresting by their votes.
Historically, it's always been the case that a stream of very similar puzzles within a short timespan will have monotonically decreasing scores.
- Someone posts Puzzle #1 based on what's then a brilliant new idea, and it gets lots of upvotes.
- Someone else is inspired and posts Puzzle #2 based on the same idea, and it still gets a reasonable number of upvotes.
- Someone else gets in on the fun by posting Puzzle #3 based on the same idea, and it gets a lukewarm reception because the idea isn't novel any more.
- Someone else jumps on the bandwagon and posts Puzzle #4 based on the same idea, and people start to downvote.
Something roughly like this has happened time and time again on Puzzling SE. A 'craze' for some particular type of puzzle surges through the site, but quickly loses its momentum as people move on to some new idea. See also A rise in low-quality questions (March 2015) and its answers - the worries you raise have come up time and time again here, and the site always survives. This too shall pass.
So how do we tackle this surge of similar questions?
Vote, vote, vote! Your upvotes and downvotes are yours to use as you see fit, and the best way to discourage a rush of unimaginativity.
Let me just drop a reminder, though, that a close-vote is not a super-downvote. If one of these 2018 puzzles is completely covered by another, then we should close it as a duplicate (case in point); but we shouldn't start closing individual questions in this class just because there are getting to be too many of them. If we want to declare all formation-of-numbers questions off-topic, then we can have a meta discussion about that and a mass close-voting spree; but, for example, this puzzle (after the edits made to it) is no more off-topic than this one and should be reopened.
are these similar enough to mark as duplicate, and be closed/deleted? Should they simply be downvoted, if not deemed interesting, in the hope that the authors will get the point?
Yes. Some few of them are actually covered by others and can be marked as duplicates; the rest should simply be downvoted until they stop coming. Don't worry; we'll weather this inrush of puzzles as we've weathered so many others.