What has Microsoft's practice been with upgrading older phone to newer OS versions?

[Windows Central Question]

I see that at least a few Windows Phones 8.1 are eligible for the Windows 10 Mobile upgrade. If I spend $550-$650 for a new Lumia 950 or 950XL phone, is it going to receive routine OS updates and and even the next OS version (11?) over at least the next two years? Are Windows upgrades and security updates hampered by carrier delays like they are in the Android ecosystem (for everything but Nexus devices)?

 

[xandros9]

Here's how the situation has played out:

2011 - Windows Phone 7 devices will not be upgradeable to Windows Phone 8.0. The iPhone of that year, the 4s is still supported and probably has the record for longest officially supported smartphone in recent memory. Lumia isn't too far behind if you start with 8.0.

All Windows Phone 8.1 devices are slated to be upgraded to Windows 10 Mobile.

HOWEVER, devices with 4 GB of memory are not guaranteed - seems up in the air at this point.

Carrier branded devices have seen the same delays and blocks Android does.
(e.g. the ATT several month delay of GDR2 + Lumia Amber for the 820, 920, T-Mobile dropping the 810 entirely, Verizon skipping over 8.1 + Lumia Cyan...)
I can guarantee that the Lumia 920 on AT&T will certainly not receive 10 officially. However, the Windows Insider program offers an alternative.

If you buy it unlocked, then I will be comfortable betting the 950 and XL will be supported for the near future. (the 2012 Lumia 920 is still kicking, it just doesn't have first dibs on the updates naturally, newer devices will come first)

However, MS has said they have more control this time around, but my stance on that is "I'll believe when I see it." They have demo'ed the ability to push fleet-wide updates as demonstrated by their anti-8.1 random-reboot fix earlier this year.