Yes, I know. It's a version of "the dog ate my homework" excuse. Only in this case it's "life ate my evidence."
The dog ate my homework is an excuse, most usually a lie. Life didn't eat our evidence, since we do not think that there is any evidence for life to eat. Prebiotic chemicals don't have the opportunity to chemically self organize and develop hereditary traits - since those chemicals will be being used for other things by existing and prevalent life.
Then, by your reasoning, abiogensis could never happen because life would simply eat itself to death. (After bug A eats bug B it starves to death, and the whole process has to start over.)
Abiogenesis would be very unlikely to occur on present day earth because present day life basically prohibits it. However, in early biotic times, life would not eat itself. Certainly Life A will eat Life B, but life B would reproduce and the offspring will eat more Life As. There would be a Darwinian struggle, you might say.