Bacteria can do the job all on its own and there's no shortage of 'em.
It doesn't just "help"; it's the star of the show mate.
No matter which creatures eat a dead one, their poo then has to be broken down by... you guessed it, bacteria.
IOW, the super-fast eaters are simply middle men, extracting some utility from what's on-offer, whereas bacteria ultimately has to clean up the mess, regardless of how many intermediate stages happen to exist in any given situation. Note that these can vary greatly from place to place, but there's always one common denominator - bacteria, making it the
only indispensable factor IMHO.
An impact, sure, but I'd argue that it'd be hard-to-measure seeing as bacteria does the most-scrupulous job.
Insect and arthropod biomass is huge, sure, but the heavy lifting is being done by "species" that're ridiculously-prevalent and in zero danger of becoming extinct. Earthworms aren't going away any time soon either.
Some context:
I've been able to achieve perfectly-balanced aquaristic ecosystems and the most-critical decision I made to reach that goal was never vacuuming substrate / gravel or even removing dead leaves. On the rare occasions a fish dies, I push it into the gravel near the base of a plant.
Point being that "science" and "big" industry in the fish keeping sector's Hell-bent on selling we hobbyists all manner of chemicals and cleaning and filtering equipment, declaring that our fish will die if we don't follow the generally-accepted paradigm. Big filtration. Big water changes. Effectively-sterile environment.
OTOH I've found that by not cleaning, I've ended up with a greater variety and total biomass of bacteria and other critters which actually help the fish's immune systems. So, minimal maintenance and healthier fish, all achieved by simply allowing bacteria to thrive where it otherwise is discouraged.
Note that I've not introduced any "intermediate" players, the equivalents of insects for example; everything's being done by invisible-to-the-naked-eye critters. These things will never go extinct as long as creatures shit and die.
Hence my assertion in my first post - The underwater environment is not only most of the earth's surface, but the bio-dynamics and critters involved therein are analogous to on-land.
Sorry for the rant.
PS:
My tanks look pristine BTW. No mulm whatsoever 'cause everything's being devoured as fast as it can be produced. Tell that to someone who spends 3 hours a week cleaning and maintaining his fish tank. Heck, I don't even run "cleanup crews" anymore (considered essential in the hobby - catfish to eat uneaten food and specific fish and / or shrimp to eat algae). IOW, and again, bacteria's taking care of everything.
Another way of looking at all this:
You could remove any given "species" or number of them and an environment would still be able to thrive as long as the bacteria remain.
If you removed the bacteria, it'd be all over red rover.