Disclaimer: I have no SaaS platform, it's a mobile app.
In my experience maintenance does not suffer long term with codebase size, if anything it became easier with model advances.
Most people talking about suboptimal implementation, fragility, and maintenance burden seem to theorize and extrapolate with no actual experience in implementing a large project with AI.
I can't blame them, I see the same in my day job, but the results after 500h+ on a separate project show that the concerns do not manifest in practice.