I can't say I have a lot of experience with this situation but here's what I'd think:
You have a codebase that is doing the job now and I think this would qualify as not worthy of a full rewrite. Doing this speculatively like this just seems wrong.
The point is to offer assurance that the partnering company is not getting a bunch of fools who wrote spaghetti code. I think if you wrote a contingency plan that outlines what you expect to happen and the changes you might make if things don't go to plan. If you demonstrate that you have good reasons for what you have done and have honestly evaluated what might happen in the future, I'd see that as pretty decent assurance.
Scaling is probably going to end up being a matter of a few bottlenecks. It might be worth maybe estimating how you might fit something like AWS Lambda to handle certain parts of the system if you need to.
Again, this kind of due-dilligence is not something I have any real experience with, but knowing that we never have all the answers in software, demonstrating that you have thought about the future and you haven't been dummies to this point would be what I'd be looking for.
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