Hi Simon,
Do those other big boys also update firmware for devices that are End-of-Sale since 6 years and End-of-Life since almost a year (I know, in June 18' X450a was still under support life)? Or do you mean just an access to the last firmware version provided?
Actually, older devices like X450a had a warranty that did not cover firmware upgrades, one had to buy a service contract to be entitled. And since X450a switch is long time after any support, it's a matter of asking the community if anybody has a firmware for the box.
I believe you are able to get updates for a second hand EXOS device same as first hand as long as its warranty covers upgrades or if you have a valid service contract (with X450a it's no longer possible to get one).
Regarding distributors... Reseller partners with Extreme directly with one link, partnership with distributor is another relationship that can change over time, when one distributor falls down nothing can stop anybody from using another one, regarding some previous posts.
Quite recent topic on second hand equipment: https://community.extremenetworks.com/general-discussion-222427/buying-used-switch-on-ebay-howto-get-updates-7575457
Hope that helps,
Tomasz
