> companies deploy them to avoid separation of responsibilities and turn every employee into easily replaceable "full-stack developer"
SPA-s are much-much harder to develop if more teams are working on it. So your first sentence makes little sense.
> companies move everything to client-side in order to reduce Amazon bills
This is never the reason why it happens. Seriously? The cost are not saved, just moved around. SPA-s are developed because they could provide a much better UX. As a side benefit, server side development becomes simpler by providing some REST or GraphQL API. You don't want to be in a place where tens of thousands of lines are generated backend side by backend developers.
> client-side scripts are poorly optimized and contribute to global warming by causing hundred thousands machines to spin up their CPU fans.
I appreciate your sense of humor :D