This seems very odd for someone to define a backend portion like this and ask you to build on top of it. The way I would approach this situation is list example

This seems very odd for someone to define a backend portion like this and ask you to build on top of it. The way I would approach this situation is list examples and situations where it makes your job harder and have reasons why a different approach would be better. If telling them that they will be stuck with these issues after you've moved on in a few years and they are ok with that then that's how it goes, nothing else can be done.