I doubt there would be enough current users prepared to do so to make that worthwhile. There would need to be at least two major changes so as to make it easier to reach new users, especially via wordpress.org, who could then support the plugin:
For both the free and pro versions, a complete revamp of the admin pages. They are what put people off using s2Member. Unlike what happens on the front-end, they aren't built to current WordPress standards and so look very forbidding and unfamiliar. Things also need re-organizing to reflect additions made to s2Member over the years, and some of the labels needs changing to be more self-explanatory and user-friendly.
For the Pro version only, either Zapier integration or a much easier API with which to integrate different payment processors.
But there is also the challenge of the horrendous Gutenburg to address. That's far more problematic because Gutenburg has the potential to completely destroy many business sites, even if s2Member is adapted to work with it. That's why I have previously said that the s2Member devs should hold off doing anything about Gutenburg until it's much clearer how the land lies. Otherwise they risk putting in a ton of work, and spending a lot of time and money, on changes that few might actually use.
To be honest, I don't see the same need for active development of s2Member that some others seem to. I think wordpress.org has conditioned WP users to think that regular updates are good. But, to me, they are just hassle unless there is something specific that I am looking for. And I don't plan to upgrade to WP 5.0 when it comes out in order to avoid the impending Gutenburg disaster, so I don't need any modifications for that.
The main reason for using s2Member is its incredible means of protecting content and files while keeping sites blazing fast and running nicely with other code. (I run 70+ plugins on my sites, yet pages currently load in less than 0.3 seconds.) I don't see any of that being undermined by changes to WP or PHP any time soon. So the only potential issues for me relate to the purchase of memberships, which are dependent on possible changes to payment processors' APIs.
Even then, I am happy with Stripe so don't need Zapier or other payment processors. And, if s2Member stops working well with Stripe, I will just use something like Role-O-Matic with WooCommerce to sell roles, while still having s2Member protecting content and files.
So a SAAS model for s2Member at this stage offers me very little. If others' current usage of s2Member is at all like mine, then a SAAS model has little to offer more generally. So attracting new users with a completely-redesigned backend seems to me to be the only way to make that idea financially viable.