What's wrong with constant ongoing evolution? Software is the ultimate metaphysical playdough. It can always be changed. Piece by piece. Step by step. Until it becomes something different. It's just... not exciting to do it that way.
I am sitting on a codebase whose oldest line of code is about 20+ years old and has evolved successfully in that time such that product it was 20 years ago and todays product are unrecognizable from one another. Its database schema is even older, encoding decisions made 30 years ago. Reasonable decisions at the time, but no longer reasonable.
I am working on a major rewrite which will take a year. The nature of the changes required mean I cannot break it up into pieces and do it bit by bit as I have been for last couple of decades with major enhancements. A functioning product is all or nothing. As someone who is anti-rewrite, pro-evolve and accustomed to working on old codebases, and being my own business so its my own money on the line, the decision to embark on a 12 month rewrite is not taken lightly.
It is a refactor, but a refactor that is going to take about a year to complete, where there will be nothing to show for it until it is finished. From old codebase, probably about 20% to 30% of it will be ripped out and replaced.
Its not my first major project in terms of keeping this codebase capable. Previously written compilers/runtimes and IDE modules to keep it going : to gain control of codebase which was written in a commercial/proprietary dev environment into something I have full control over. That effort took a year.
The difference in this case is I am deliberately, intentionally throwing code away, alot of it, for the first time and contrary to my instincts. Still a post mortem might be interesting. My successful efforts to code my way out of a proprietary dev environment was an interesting and risky project, discussed at length with relevant dev communities at the time, but probably worth writing up one day.
With regard to your own project, I find it somewhat hard to believe that you can rewrite a 20 year old code base in just one year's time. Assuming an output of 50 LOC a day - and 250 work days a year - which means the system you are replacing is under 15k LOC.