I think if a post is deleted for whatever reason then people should lose that merit as well. I think some people are purposely deleting their merited posts after they've received merit for it for whatever reason. Anyone can just merit some random trash post then delete it to get rid of the evidence. People could also just copy content then deleted the post once it's been merited. It's ripe for abuse if the merit stays after the post has gone.
I agree with your point but will add a secondary opinion:
For a post to be deleted means that it is not meritorious.
Whether the thread be trashed or the post deleted, it simply means that the post does not contribute to a discussion. This may not be the case for all posts but should encompass most deleted posts.
All the more reason to remove the merit for such posts. Simply means that they're not high-quality.