All of that, while technically correct and possible, is vastly more complicated and time intensive than a rewrite of what the OPs description of the codebase is.
Yes, it absolutely is - but the trade off is a far lower risk of introducing breaking changes. Depending on the industry/market/clients - it may be the right tradeoff
In my eyes, a rewrite won't be introducing breaking changes. It would be to figure out what functionality makes money, then replicate that functionality as best as possible so that the company can continue to make money as well as build upon the product to make even more money.
We're talking about a webapp here, not rocket science.
