You are right, there isn't one. But there is definitely musical substance there, as opposed to just a style/marketing lable.
Which then leads to my bigtime pet hate. The fucking purism of forever locking Grunge as a term only describing one defined set of bands, from one specific area, from one determined time period. And that nothing else may be called gurnge, despite actually sounding it and having the qualities of grunge. I honestly don't think any other genre has that rigourous elitism. And that's where part of your point is actually quite valid.