I can confirm that the Mendix debugger is at least buggy. It probably crashes about one in five debug session for me. I have been experiencing this with I think 5.20 and 5.21.2 on Windows 7 and 10.
Does it respond if you only kill the java service, because the modeler is actually not running on java it is the MX runtime that run's on java. The modeler runs in a separate process. So it might be in combination with the project that it crashes.
Does it also crash if you debug server's like connecting to a cloud node for a debugging session?
I never had any issues, but then again it might just be that your project is a lot larger.
The size of the model is in my opinion not the issue. I think we have one of the largest applications (currently about 240 mb) and have not those issues. We have had some bugs where the debugger would not show some objects but for the rest I have no issues with it. To rule out the option of a specific model does it also happens when you create a new test project? In my opinion it must be something machine specific. Did you try it on another machine?