You are not writing any of those systems, you're merely writing the glue code between your application and those systems, none of which is application specific.
I explicitly made the point that cross-infra porting is not necessarily trivial, but it is also not cost prohibitive as the OP makes, and certainly there is no secret sauce involved to the extent of justifying a break-up.
> except for some of the business logic
You mention this in the passing, but barring mechanical limitations of the new infra, reusing your business logic means retaining your APIs, auth models, data models, even database schemas, replication and deployment design decisions and tons of portable code. That is where the overwhelming majority of engineer hours go in an ordinary product anyway.
> three do run divergent platforms simultaneously, today, right now
No, they don't. Trying to spin up your custom internal infra (or demanding special internal accommodation from externally available infra) will require justifying to management why you're not using the existing one, and most of the new products coming from these guys are nothing that specialized in terms of infra needs. I'm not saying there is a perfect external/internal service&API parity, but certainly convergence is the normativity here, not divergence.
Let me put it this way, majority of {Google, Amazon, Microsoft} services could be ported to run on each others cloud platforms, let alone on the external APIs of their own platforms, which means there is no secret sauce and there is no reason for smaller players to be complaining about it other than not having economies of scale thus pricing levels.