The main reason it may not last is possible it takes time and resources to continually develop and release 4 new cards each month. As far as we know the 4 cards that have been released each month could have been cards they wanted to have in from the start of the game but were unable to finish or felt that they needed changing for whatever reason so they moved them into a monthly reward system instead. As well as part of the problem of the monthly cards seems like most of them seem to get nerfed/changed (like Keeper of the vale and Jaxi which you mentioned in an earlier post) or see pretty much no play like Envybaer, Quartermaster Gauj, Ironclad, The Scientist. Which may also suggest that they are not play tested enough or trying to make too niche of a card text to be useful.
Plus there won’t be any new cards this month. One of the recent patch notes said that this month there won’t be releasing new cards but instead it be cards from the relevant rarity. So if you reach gold you get a random legendary plus the silver rank reward and so forth.
Anyway, I’m agreeing with you that releasing new cards per month helps somewhat against the game being stale, its a nice reward system, it sometimes changes the meta, it can add in useful cards and that CP are more willing to nerf cards.
However, the topic was talking about quarter expansions and I’m comparing it to Hearthstone because its arguable the most recognizable/biggest game within the same/similar genre which also has quarterly expansions that vary in size from 100+ cards to the adventures which is more like 30-40 cards. Which are similar to the 2 expansions we have got from Duelyst so far (minus the single player element of adventures). There are differences between the two games but the comparisons can be made and the likelihood that can come from that.