There are multiple ways to deal with it from an engineering point-of-view but the conversation is a business one. Is the product in its current form likely to b

There are multiple ways to deal with it from an engineering point-of-view but the conversation is a business one. Is the product in its current form likely to be in-use for many years to come? That is different from a product that is only used until the next large raise in maybe a year or two in which case you can maintain it until the next release. That said, one of my favourite ways of "rewriting" is firstly to identify any parts of the system that could be separated off into a second application and start designing and building a new system in parallel. Another relatively easy task is to spend a small amount of your working week building the "rewrite" from scratch starting at the highest level so that you can learn some of the problems that are not obvious from the current system but which the previous developers had to learn while they went along. You could build a large part of the scaffolding without struggling too much and then have the basis of something to suggest for the future.