So answer the question — is the Nexus 6 obsolete because of the processor?
Nope.
When you can buy the Nexus 6 and hold it in your paws, it won't make much difference at all. We won't see the "dramatic" battery life improvements that we expect to see from ARM v8 chips, but the things you run and the way you run them won't change because Lollipop offers 64-bit support.
It's just not totally future-proof.
When you can buy a Nexus 6, the processor won't make much difference.
One day, Google will stand on stage somewhere to show off some amazing new feature in an upcoming version of Android that will only work "if the hardware supports it." That will be code for "We're leveraging the extra computational power of a 64-bit processor and it would lag like hell on a 32-bit processor so we won't offer it to those."
Chances are, the Nexus 6 will be long in the tooth at that point and everyone who bought one will be ready to buy new hardware already. But there has to be a cut-off point if Google ever hopes to use some of the features of the 64-bit architecture to write more intensive and power-hungry software. We assume Google is smart and will make that cut-off point happen once all the OEMs (and themselves) have had devices that can support new features available for a while. But we can't be 100-percent sure.