I would say you can’t, not possible. There is no filter for get_the_post_thumbnail_url() or get_post_thumbnail_id().
However you could apply a filter to get_metadata() which get_post_thumbnail_id() uses, and return the id of another attachment that you want to use as the default thumbnail. But I don’t recommend this it’s a bit hacky.
I think you should refactor your code and just make a function to check if get_post_thumbnail_id returns anything.
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