From a business perspective, nothing is broken. In fact, they laid a golden goose.

> team is 3 people, quite junior. One backend, one front, one iOS/android. Resistance to change is huge.

My mistake, they didn't lay a golden goose--they built a money printer. The ROI here is insane.

> productivity is abysmal which is understandable. The mess is just too huge to be able to build anything.

But you just told me they built a $20M revenue product with 3 bozos. That sounds unbelievably productive.

> This business unit has a pretty aggressive roadmap as management and HQ has no real understanding of these blockers

You should consider quitting your job.

As far as the business is concerned, there are no problems... because well... they have a money printer, and your team seems not to care enough to advocate for change. Business people don't give a damn about code quality. They give a damn about value. If 2003 style PHP code does that, so be it. Forget a rewrite, why waste time and effort doing simple refactoring? To them, even that has negative financial value.

From their perspective, you're not being paid to make code easy to work with, you're being paid to ship product in a rats nest. Maybe you could make a business case for why its valuable to use source control, dependency management, a framework, routing outside of nginx, and so on... but it doesn't sound like any of that mattered on the road to $20M a year, so it will be very difficult to convince them otherwise especially if your teammates resist.

This, again, is why you should consider leaving.

Some developers don't mind spaghetti, cowboy coding. You do. Don't subject yourself to a work environment and work style that's incompatible with you, especially when your teammates don't care either. I guarantee you will hate your job.