Well, seems to be higher purpose that 'just evolved and that's it', anyway, so I would probably take it if I were animal kingdom. 
I kinda' hope you're kidding, but I'd definitely take "meaningless" over "the laughter of hungry gods" any day of the weak. Better to have no purpose than that purpose to be tortured, killed, and eaten. The latter is kinda' like saying that you should be happy to look forward to a future of being raped for the rest of eternity -- at least you had a purpose, right?
Bugger that with a cactus. A terrible purpose is worse than none.
Also, what is a food system that doesn't tie into the major ecosystem?
Most of the deep-sea stuff is largely unattached to the larger ecosystems. There's plenty of extreme condition systems that are effectively disconnected from everything else, as well.
But since it is generally seemed as bad thing to make species go extinct, I presume they all fulfill some role in ecosystem? Or is just for the sake of 100% completionists of real life that we try to save the endangered species?
It's mostly because the so-called equilibrium you're talking about is
incredibly fragile and often goes off the rails all its lonesome. We want biodiversity because that means it's somewhat more difficult for the entire system to go to pot when half of it can die off and the rest still make it. Because stuff like that
has happened before. If YWHW was a bioengineer, it was a goddamn
horrible one.
Actually, we are perfectly capable of eating any animal I know that can eat us... They just part of the ecosystem.
Plenty of parasites and whatnot that are perfectly capable of killing us and living off the remains that we can't do jack about, much less
eat. Bacteria rule this world by the numbers, not man.
It is stated in the Genesis, that the animals were created for humans; see Genesis 1: 24-30, for starters. So this is written in the Bible, and thus we know it.
Man, if rulership means you're supposed to eat your underlings, our monarchs and whatnot have been massively heretical.
... regardless, those verses do not mention the consumption of animals
at all. YWHW actually pretty specifically states we're supposed to eat plants in those lines.