That's one plausible interpretation. However, I'd like to offer a different--and perhaps simpler--explanation.
Perhaps Blizzard is removing the Upgrade Vendor NPC in 5.2 so that they can bring him back in 5.3.
See, the thing is that--given Blizzard's current goals for patches--5.3 is probably not going to include a raid. I imagine that it will be a patch very much like 5.1 was. A new reputation, a new storyline, lots of random side-events. Those sort of activities only ever reward sidegrades. They won't offer true upgrades a tier higher than the current raids.
Of course, character progression is still important. I think Blizzard still believes that a patch without any way to improve your character will not be popular with their playerbase. That position could very well be correct.
Bringing back the Upgrade Vendor in 5.3 allows people to continue improving their gear at that point. You continue raiding to fill out missing pieces and earn Valor to improve your current gear.
This is a much more likely plan for Blizzard. Upgrading gear probably required a significant amount of tech to implement, and it is very unlikely that they did not foresee any mechanical issues (with scaling, etc.) that would result.
So I think the plan looks more like:
- In 5.2, players upgrade their gear by doing the new Thunder King raid and getting new items.
- In 5.3, players upgrade their gear by spending Valor on the gear they earned in the previous tier. All the non-Royalty guilds will also still be progressing through the Thunder King raid, and earning new gear that way. And of course, filling things out with sidegrades from the new non-raid content.
That's more plausible than Blizzard suddenly deciding right now that the entire concept of upgrading gear was a mistake.