I would love to get to the point where we have so many people actively maintaining each track that nobody, ever, feels any twinge of guilt at moving on when they've had their fill.
Matt has added to the Java Track in just about every way. One of our earliest interactions was on the Java track itself as we found a common curiosity exploring nucleotide-count, looking for what performance we could get out of the Streams API — that was ridiculous fun for the both of us. His first PR was adding pangram to xJava. Today, he's a top 10 committer. More recently, Matt has been a skilled mentor for contributors to the track, exemplifying the way we are, here at Exercism: treating every one with respect while holding them to a high bar of quality (that's just how he is).
For eleven months we ran what we referred to as "The Matt & John Show": a video hangout where we tackled all kinds of challenges on Exercism. We dipped our toes into machine learning (specifically natural language processing), mapped out all parts of contributing to Exercism, and did our best to bring some structure/organization to the track. If you benefit from structure around here, know that Matt had a solid hand in that.