I want to contribute the discussion from a new player perspective (and with consideration of mobile in the future).
CPG released Ancient Bonds prior to whenever mobile will come out from beta. This being the third expansion to come out, the power of expansion cards needs to be watched (this does not mean addressed).
I'm trying to play budget decks, and it's tough. I often feel disadvantaged and discouraged. My motivation and enthusiasm drops as the number of legendary and expansion cards I come across increases.
The addition of another means of playing (mobile) will naturally allow a potential increase in player base (population), but (as @Ryvirath touched on) retention is a topic that isn't answered by mobile; but is a separate, and important, topic.
Now CPG obviously needs to juggle multiple opposing things like profit (charging for orbs/etc. and/or the difficulty of achieving them) verse player population.
The player population WILL be impacted when mobile and new players come around, but the retention of said new players will ALSO be affected by how they feel walking into matches against players with 1 or multiple expansions.
What's this balance of allowing and providing an environment that encourages new players I don't know. If the power levels increase as expansions continue, does it increase or decrease the likelihood of overall profit and player population?
I'm struggling to say what I mean. To provide an example, I played Vet vs Vet, I was budget, they had Ancient Bonds. The fight was overall a stomp against me. I used 2 entropic decays, but EMP and Blood of Air were played and closed the game of any hope. Blood of Air is an almost strictly better card that was available to my opponent and not me.
I present this because the question has to be brought up, "How is this going to be for players when we release mobile?" This is a serious question, because this WILL be an EXTREMELY common occurrence for those new players when they try out the game. Some will react by paying, some by leaving, some by effort/diligence. But to return to some original point(s), if the amount of money and expansions required to feel like a fair-ish fight.
The later mobile is released prior to expansions, the more it is going to feel like an uphill battle for potential new players, and thus, increase the likelihood of them leaving, leaving only a similar population trend that was had on Ancient Bonds release.