I wouldn’t say Martin’s shied away from the warden concept, no — it’s just that as is often the case in Westeros, while de jure titles carry a lot of weight, they don’t mean nearly as much if they’re in conflict with a de facto claimant on the same institution. At the same time, the realm has fallen apart, so East West North and South don’t matter as much as they used to. So no, the Lords of the Vale are not gonna ditch House Arryn for Jaime, unless and until Jaime starts defeating various lords of the Vale.
As for Jaime inheriting the Warden of the West position, it’s not a castle or a lordship, it’s a title designed to ensure the realm’s loyalty to the monarchy like Hand of the King, which Kingsguards can and have occupied. And it’s been in the hands of House Lannister since the Conquest. There’s no reason to change that now.
