My main problem with adding new guns every season is I don't think they can remove or rotate out weapons when people are paying for skins. Things start to get cluttered relatively quickly if they truly intend to stick with this game for the long term.
If it ever came to the game having too many guns and weapon balancing became an issue I could see them doing something similar to what Hearthstone does with its Standard and Wild format. In this case unranked could have all the weapons whereas the ranked season would have whatever the current set of weapons is with certain ones rotated out. Or they could have ranked and unranked the same with another 'unbalanced' playlist with every weapon/hop-up etc there has been gets thrown in there.
I don't think this will be an issue for quite some time, if at all, though.