yeah that could be a possibility! it’s certainly something i hadn’t even really thought of before, and it does make sense in terms of Crowley’s character that he would still want a power advantage. because i mean if you think about why he wanted the souls in purgatory in the first place, to give him power to cement and exert his position a king of hell, the only reason he ever really roped Cas into his plan was because it was an opportunity he saw for an advantage and he seized it, every bit the vulture. similarly, when analysing whether he should side with Dick Roman or the Wincesters, he looked at who he could have the most advantage with, and seized it.
but by that token though, i have to wonder if Crowley would really find the souls in purgatory worth the trouble? I mean he saw what they did to Cas, they sent him out of control, and Crowley wants to be in control. and really with Dean and Cas locked in purgatory, and Roman dead, Crowley has a huge advantage on earth. he now has no great immediate threat to his power to deal with, except Meg maybe, and potentially Sam—although I doubt Crowley really sees Sam as a threat at this point. i feel like Crowley’s more going to try to capitalise on having Castiel and Dean and Roman out of the way than try to chase after them in purgatory to get at the rest of it.
but no doubt there are going to be bigger plans with him!