So is 1000 people trying to coordinate package versions and interface changes across 100 systems (throw in loosely typed languages for extra fun).

That assumes all services talk to all other services directly. a) they probably don't, and b) message busses can help a lot.

APIs shouldn't change in backward-incompatible ways. That's sorta the bedrock of a service oriented architecture. If teams have to communicate with each other through more than API documentation and can push responsibilities onto each other by making backwards-incompatible changes then you've kneecapped the entire benefit of a service oriented architecture from the get-go.

I'm not talking about backwards compatibility so much as adding features in parallel that span many services, which happens when you're closing a lot of sales deals when the salespeople don't say no to things :p