There is some truth in what you said. Although Nexus owners would never admit it, the 6P is a developers device and yes a test bed for Google. But both Google and this particular model phone have reached such a high level of quality, you would be hard pressed to know it. It is exceptionally good. Android M is just a medium bump up from L that it was easy to make it work well. How quickly people forget the nightmare of the Nexus 5 update to L. It is a legitimate buying consideration to recognize that the next update to N might not go so well for Nexus owners and that is part of owning a Nexus.
On the flip side, things have changed in security concerns for smart phones, so to answer your question about why people worry about upgrading immediately, there are very good reasons now to want an update. The reality is, even Googles security updates for Nexus devices are slow (monthly) by PC and Server standards and still keep a lot of devices unprotected for to long. In comparison to LG, which they rarely, if ever do a security update at all, or at least one that manages to get passed along through a carrier. This alone is a legit reason for buying a Nexus over LG and risking a botched upgrade down the road. Toss in that it is one of the best phones out currently, your argument doesn't stand up today. It is an old outdated argument.
There are people who buy the V10 that absolutely bought the right phone for them, because it is very nice and well made, but you can't toss the Nexus people under the bus for wanting the latest updates and upgrades.