Just got the update. Phone is restarting now.
As for why I need to keep this phone for no less than 4 years its not because of an actual contractual obligation.
My dad just drilled into me the importance of maximizing the utility of everything I buy and since Google is now supporting Nexi for 4 years (2 next OSs and 1 additional year of security patches) I can't justify upgrading early because the phone still works well (except for the battery falling apart).
As for why not just get a replacement battery the problem is I have no background in electronics whatsoever. Thus I'm unwilling to risk permanetnly damaging my phone by trying to take it apart to replace the battery.
Depending how 'active' you are with the phone, you'll see a sizable to enormous increase in battery life (in standby, drain is negligible with Doze).
Keep an eye out for anything that legitimately represents a condition that is failing under warranty while you're still within your first year (or even second, if you paid for it with a
credit card that extend the manufacturer's warranty - a fact most people forget or are unaware of, and a value provided under most credit cards. If you have an issue covered in that time, you're likely to receive a replacement that would have a new battery (albeit the device itself might be refurbished).
In the future if you're going to be holding on to a phone for more than 2 years, I recommend waiting the 6-8 months or even year after release until the device takes its first price drop (consider that the N6 can be had now for $300). Doing so won't have you enjoying the 'latest of the latest' at any point, but over that long of a term it would be a shortlived thing anyway, and you're cutting your effective cost in half.
As to the battery itself, I'm surprised you're having issues after a year of use. If it's conspicuously bad, you could try contacting Google if you bought it from the Play store - even if it's not directly covered under the warranty, they're likely sitting on a large stock of refurbished units since they have still managed their own warranty service for the N6, and you might catch a rep who willing to accommodate a replacement. While it's reasonable to expect some degradation of life, you should, even with daily charging, be retaining probably 85-90% of original life. That said, I've yet to have owned any Li-Ion/Li-Po battery offer reasonably functional performance beyond about 3 years.