I usually am in favor of a complete rewrite. I'd also prefer to not grow projects into multi million line monoliths. Just make multiple smaller ones that can interact independently with each other. Much simpler structure. Also safer in the long run.
You actually believe that? Distributed systems are simpler than monolithic? In PHP?
This business wouldn't exist if they attempted to follow your advice, because they weren't able and anyway didn't have the money to hire that many developers. There were a couple of subsystems they tried to implement the way you suggest, e.g. one for running certain background jobs.
It was a database table with one row per type of job and a little metadata like job status and a copy of the input. They started jobs by sending a HTTP request. This was a constant source of manual handling, because things started jobs and then crashed and never reset the status and things like that. You could respond that they should have used a message queue instead and so on, but the thing is, they didn't know how to build reliable distributed systems. Few developers do.
