That “main engineering effort” will go on forever. People neglect to note that everything is constantly changing. Just like the roof on your house, if you don’t upgrade your components regularly, eventually you will face a huge rewrite when that thing your ancient home-made infrastructure relies on is no longer supported or is no longer updated to support the latest thing you need for your SaaS.

You can’t avoid this cost. Some people refer to it as technical debt, but I think more accurately it could be called “infrastructure debt”. Platform providers maintain the infrastructure debt for you - this is what you pay them for. And they do it with tremendous economies of scale. Unless your scale is truly enormous - like Meta, for instance - it isn’t worth build your own infrastructure.