You as as a startup should be fully aware that a big player can enter the space at any time they want. This is not Amazon's problem. It's your problem.

Also, it's a flawed abstraction to view large companies as a monolith: the investing arms are usually very separated from product teams.

TLDR: If a large company wants to enter a space, that's their choice. Accept this as a fact of life if you want to go work on a startup.