Debugger integration has always been the killer differentiator for IDEs.

I think that's a pretty bad choice of comparison. For one, they're two products under the same company. Secondly, Visual Studio is a bit of a dinosaur that suffers (and somewhat benefits) from a long history so it's easy for the VSCode team to look at that and just trim the fat and correct mistakes. They also target somewhat different domains with Visual Studio having a longer history in C++ and VSCode being more geared towards web dev (yes I know it's widely used for other languages but web is the leader and obvious target). Visual Studio is also by far not the best in its class, using IntelliJ products side by side with Visual Studio is like night and day especially when it comes to C++. A better comparison for VSCode would be something like Sublime Text since it's in the same product space of "lightweight" code editors with a focus on web.