https://en.wikipedia.org/wiki/Conway%27s_law
The law is based on the reasoning that in order for a product to function, the authors and designers of its component parts must communicate with each other in order to ensure compatibility between the components. Therefore, the technical structure of a system will reflect the social boundaries of the organizations that produced it, across which communication is more difficult. In colloquial terms, it means complex products end up "shaped like" the organizational structure they are designed in or designed for. The law is applied primarily in the field of software architecture, though Conway directed it more broadly and its assumptions and conclusions apply to most technical fields.
(Strictly speaking you can have a web service that's purely about spitting out raw data in a form that other organizations can use, but that's the kind of thing that Semantic Web and Linked Data standards are intended to address. Not something that your garden-variety website has to be concerned with.)
Without that knowledge, I agree that good old SSR (MPA) is easier and more maintainable. And more robust.
The only reason to still stick to an SPA instead of an MPA is that the app is so bloated you want the user to only have to load it once.
And people say "lazy loading!" but by the time you've implemented lazy loading in a SPA, you could have just used an MPA to keep things modular and memory efficient.
SPAs became popular at a time when companies thought everyone was going to move to progressive web apps. Then the bloated frameworks came along, PWA interest has faded, and here we are.
The above are all my opinions, apologize in advance if they come off as speaking objectively.