Because we talk so much about our back compat efforts, it is certainly understandable that people might think that is where almost all of our development effort goes. But that isn't actually the case.
We heavily emphasize our philosophies because it makes adopting and updating WordPress easier, and we want users to know that. But in practice, the way we build software doesn't hamstring us. We've introduced new features and rewritten entire APIs without needlessly breaking plugins or sites. And we do sometimes drop stuff -- we are just careful about it. We do whatever is necessary to make WordPress evolve. In the end, we've simply become really good at following through.