I visited a fascinating museum with a ceramics collection, and it included an exhibit with a history of ceramics. Earlier ceramics had this problem. Apparently, bone china -- with some bone mixed in -- created a tougher material that was less prone to cracking under the rapid temperature shock. As ceramics improved over the decades, it became a non-issue.

I suspect that letting the heat spread out over a larger area at the bottom of the cup might have alleviated cracking. That's a speculation of course.

I wonder if this is why the Russians drank tea from glasses. My mom has some lovely Russian tea glasses with silver holders.

When you pour hot tea into an empty cup, the innermost layer of the cup goes from room temp to ~80C within a fraction of a second.

When you pour hot tea into a cup that already contains a fair amount of milk, it goes from $milkTemp to ~60C gradually over a few seconds as the tea mixes with the milk, giving enough time for the cup to expand evenly.