It will definitely be interesting to see if Apple releases a new one down the road, though I'd wager they won't.

To be honest it has been several years since the Thunderbolt Display has been even somewhat competitive. I feel like it has limped along purely due to people buying it to maintain a cohesive aesthetic among all their home Apple stuff.

I have been reading blog posts about implementing vertical rhythm in CSS for years but have never seen it implemented on any big sites. Why is that? Are people just putting it in the "not important, too hard" basket?

Me too. I love its origins, I love its versatility and I love how despite how often it is imitated, it remains so unique and identifiable (the same can no longer be said about, say, Gotham).