At some point a partial rewrite might become a complete rewrite. It depends. Ship of Theseus[1], etc.
- remove one bolt and raise in the air
- slide out rev 30 "parts" from beneath bolt
- slide in rev 40 "upgrade parts" (fuselage, wings, engines, etc) underneath the bolt
- fly "upgraded plane" without a lot of pesky 'new plane" studies
Additionally, in parts of california, how to build a house:
- find existing house
- pick a wall
- remodel everything except for that wall
- reap tax benefits in your 99.9% new house.
Gradually re-written from Java (JBOSS) to Python over about 10 years. Basically the Python side knew what URLs it could handle and proxied the rest over to Javaland. We ended up shutting down the last Java bits early last year.
Partial rewrites based on Domain/Model/REST endpoint and just proxying to both webapps based on which was new already.
No breaking changes from the outside, either share the parts of the code that are still good (most of the business logic might be) or fork them (and then refactor and maybe you need to fix bugs twice) and after a while you can switch off the old part.
Works like a charm with added benefits of being in the same language to avoid wasting time. But the base idea works even if you use different languages.
It's also not for web apps, for example Apache Storm can do Bolts? (been a while) in several languages so you can also easily rewrite parts, if you can serialize your data in and out of it.
I would argue that if you already have a lot of in-house knowledge with Java then you might as well stick with it (and just not make whatever mistakes you made last time around) but Python seems like a reasonable alternative to me.
At the time the re-write started, Django hadn't yet seen a public release, Rails barely existed (and had zero traction, and Ruby-the-runtime was horrible), dotnet was barely a thing, Clojure and Go didn't exist yet, dotnet barely existed. 2005 was a weird time.
Realistically the only options at the time were Python, PHP (4, not 5), or Perl.
I didn't come in until much later, but I don't really think there was a better option at the time.
do you actually know what that would mean?