Only if the main job was to eat plankton or squid or something that a fish could do just as easily.
I have to admit a little confusion to this part of your statement. It may even be nitpicky, but describing the ecological niche that a species has as a 'job' seems to suggest some kind of ordination. Whales, through the process of evolution, came to exploit the niche they occupy. This ecological niche wasn't handed to them or offered to them by some job placement agency.
But, if the job was also to be eaten by a host of other deep ocean creatures and spark deep sea echosystems, then a very large deep ocean creature performs that role better than a manta ray or a whale shark.
That isn't apparant at all. It happens to be that they can be a rich resource, but within the immensity of the ocean, there is a lot of competition to find these resource pockets and traveling from one dead whale to another is no easy business. Whales travel over thousands of miles and those species that feed on the dead carcases of whales must travel to them, through some pretty barren areas. A god would have made things much more efficient of he split up the 'job' of a whale between a multitude of creatures. That would have made it much easier for those poor creatures at the bottom of the ocean.
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