I am a software engineer who's been involved in the tapeout of a few ASICs (although none of the TPUs). Particularly when you plan to build a series of chips, the continuous approach taken by software is massively preferable. X v2 does what X v1 did, plus some additional things, and with all of the errata fixed. Also, you find the errata in X v1 after tapeout but before your driver team does, saving them an enormous amount of work trying to track down a driver bug that's actually a HW bug (maybe even one with a simple workaround).