> We did a full rewrite and it was quite successful and the company went on to triple the revenue.

Which sure beats some other company coming along and "rewriting" the same or similar functionality in a competing product and killing your own revenue. But it does come down to how big the codebase is and how long it would take for an MVP to be a realistic replacement. If there are parts that are complex but unlikely to need changing soon you can usually find ways to hide them behind some extra layer. Is there any reason you couldn't just introduce proper processes (source control, PRs, CI/CD etc.) around the existing code though?