The
https://www.ifixit.com/Teardown/Nexus+6+Teardown/32877 doesn't look very easy for a battery replacement. I'm not saying it's not doable, but not a five minute job.
I hope the battery last the useful life of the phone but new phones (Nexus 6p for example) are incremental not revolutionary in terms of improvement. I see (for me, YMMV) no reason to upgrade to the Nexus 6P.
Nexus phones, similar to other manufactures, have their good and bad points. The nexus non-removable battery is a downer.