Some thoughts:
Personally I would not want to install another bloaty (Steam) client just to be able to use TripleA. But as long as TripleA will be kept as an independently installable software I would not object.
Concerning the user base: I have no idea if and how the user base would enlarge. But if it did it would mean, that the need for support and explanation would increase, too. Now who will cover that? Who will monitor Steam-forums and maybe Discord-channels?
On A&A .org, the PBF-base of a remarkable user-base, it is basically me as a one-man-show who helps users with troubleshooting in case of software problems on their side.
And I must admit that - at least since 2.6 - development - this "job" has become harder for me as 'improvements' of the software to me have become more and more unclear. Not a complaint - just a remark that giving support to me becomes harder and harder, too. I am not sure how long I will be willing to continue this.
Before thinking of enlarging the user base by using one of the popular game distribution sites one should reflect what is feasible not only concerning the possible consequences - but also concerning the current lineup, too.