Yeah, that part was fascinating. I knew from previous episodes that the Tardis was alive, but I'd never considered what would happen if it died of old age. I always thought that was a little out of range.
I'd "seen" a dead TARDIS turn giant before, in the 2002 Big Finish audio drama "Neverland," only there it was discovered from inside the giant console room. It definitely pleased (and amused) me to see the TV series follow suit.