The software engineers have plenty of leverage over every company without unions. A proof of that is high wages across the industry. And for Amazon in particular, where is that extra money coming from? It's not like they have a dividend to shareholders they could stop paying out. They're not some law firm where the partners are profit sharing. It's a publicly traded company that just constantly reinvests in growth. They're practically famous for that. So really to pay more, they must either get more efficient (probably already trying at that) or raise prices or stop reinvesting. I guess they could pay the higher paid workers less, but isn't that already the software engineers we're talking about? Those are the higher paid workers.
As an aside, I sometimes wonder if they could literally have qualified as a not for profit whose sole goal is to grow into new market segments and employ more people. Because if you take a wide angle view of it, that's pretty much all they ever do.