For having owned 7 Motorola devices you are rather uninformed about recent Motorola updates.
First, only 2 models of the 2014 Moto X did not receive 6.0. They happen to have been the 2 US carrier versions ( Verizon and AT&T). Those 2 models DID receive 1 update however, kit Kat to lollipop so you are incorrect that they received 0 updates.
However, the Unlocked version of the 2014 X (2014 Pure) has received marshmallow. Not only that but the 2014 pure has had 6.0 for over 2 months now, which means that Motorola's 2014 flagship beat the majority of 2015 flagships to marshmallow! (notable exceptions being Nexus and the 2015 pure) Also consider that Motorola's 2014 midrange phone (Moto G) is now receiving marshmallow, which means that Moto's 2014 mid range has still beaten many manufacturers 2015 flagships to 6.0!
So yes, while I do feel bad for those who bought 2014 carrier branded Moto xs, there is ample evidence that Motorola has and will support its unlocked devices in the future.
There is no arguing that Nexus will continue to get quicker updates than Moto. However the statement that the 2015 pure has received its last software update is demonstrably wrong.
Posted via the Android Central App