It'll last about a week or two with numbers back to normal by about a month. However, the baseline numbers has gradually been increasing. Since the netcode patch things were looking up. Then MM hit, but for all the vets that have stopped playing as much, more noobs fill in. When the new weapon is added and if certain ship refinements happen, we'll likely see a resurgence in veteran crowd. Ultimately tho, if Muse can get enough people into GOIO now, they may be able to get some bigger sales for COOP when it hits.
This is something I noticed Dino D-day did with their DLC COOP mode. They lowered the price of the initial game to super cheap for the steam sale then left the DLC price higher. The game is fun enough that I've been tempted to splurge and get the COOP addon. Trouble is, no one I gifted copies to has gotten into game yet. When there is no friends to COOP with, there is no reason to buy the DLC.
Awesomenauts has done a similar thing with Starstorm. They cut the main game price to dirt cheap then leave Starstorm high. Cept to me they haven't given a reason to pay the higher price yet. Course I'm not a serious player of that.