Amazon is filthy rich and is investing heavily in the entertainment industry so it's the perfect partner imo!
That's the problem though: they're not invested in the entertainment industry — they're invested in cloud services.
Lumberyard is a means for them to sell AWS. They have no interest in the engine beyond that. If the AWS sales don't pay off, they will kill it off instantly because it's worthless to them. They're filthy reach because they don't keep money-losers around, which means that even legacy support isn't really worth it. That effort is better spent on a new and improved way of selling the same AWS.
Put another way: Amazon is not a game engine company. Unlike a company that lives and dies by the use of their engine, they have no incentive to keep a game engine running until the bitter end — only until it dips below “saccharine sweet.”