[tinco]

Well you're not asking me but this is a forum so off I go anyways. This can only be done with serious investment, so you need to ask your management if they are serious about the long term future of your product. If they are there's basically two options.

The first option is to develop an API layer over your old system, that is supported by a full integration test suite, and then either build a new frontend using that API, or port the old frontend over to that new API. As this effort progresses, your integration test suite and well defined API allow you to replace/rewrite portions of your backend as well.

The second option is to look at your most valuable customers, discern what features they use and rely on, and design a fully separate clean slate implementation of only those features in a new "V2" product.

In either case, what you're describing is a very tough situation, and any solution would be very risky. Management usually is not aware of tech debt, they might not know they've accrued a million dollars or more of engineering debt that they will have to pay off to solve problems like being able to quickly respond to new market developments, retaining high quality engineers or ballooning customer dissatisfaction.

[aspaviento]

Thank you for your feedback. I'm also in favour of unentangled frontend from backend as a first step. Problem here is that the frontend needs to be rebuilt in a different language and $boss wants to keep releasing new versions during the rewriting (obviously) so porting the old frontend allows to release new versions but doubles the time for the rewriting while building a new frontend would leave the customers without new updates for a long time.