Thanks for the question! We always want to keep our devices on the most up-to-date SW releases and we work hard with the Manufacturer to ensure the updates are received in a timely manner. I know that we will always support your device for the duration that you own it but as far as a 4 year update road map, that is not something we can guarantee as there are many factors that come into play when a device received an update. While it may not be feasible for the S9 to get OS updates for the next 4 years, if there is ever an important security update that needs to be rolled out, we will be sure your device gets it, no matter how old it is.
Understand that there are lots of moving parts between the carriers, manufacturers and the OS developers.
I have not been real happy with the long term support of 2 smart phones procured from T-Mobile, the Google (HTC) G1 and the MyTouch 4G Slide (also HTC), nor from smart phones bought direct from the manufacturer or other supplier. I still really like my G1 and 4G Slide, BTW. And frankly, Samsung is notorious for lack of long term support, but maybe Project Treble will help. Time will tell.
I miss the days when agreeing to a 2 year carrier contract included a new free smart phone, where free is priced just about right. ;-)
$700+ for a smart phone is a bit too pricey for me (and many others).
Now if T-Mobile offered, say a $50 per year discount for each year of a continuous T-Mobile account, the Samsung Galaxy S9 could be right back on my purchase radar.
Here's an example of what I'd (or we'd - as in T-Mobile customers) like to see regarding future support for the T-Mobile Store Samsung Galaxy S9, as noted at:
o Pixel 2 (2017) phones get Android version updates for at least 3 years from when the device first became available on the Google Store. After 3 years, we can't guarantee more updates
o Pixel phones get security updates for at least 3 years from when the device first became available on the Google Store, or at least 18 months from when the Google Store last sold the device, whichever is longer. After that, we can't guarantee more updates.
o Pixel phones get telephone or online support for at least as long as they get security updates. After that, we can't guarantee continued support.
Note that my understanding is that the Pixel 2 is made by HTC and the Pixel 2 XL is made by LG, but Google is taking responsibility for the support regardless of the manufacturer.
This Google Android support roadmap makes me more comfortable than what little I've read about the support roadmap for the T-Mobile Samsung Galaxy S9.
Not the 4 years I'd really like to see, but at least 3 years is nice.