You don't need to do a deep analysis to tell if optimization is needed or not, it is enough to assume best-case improvements and compare it to your FTE + overhead cost.

Do you have many of your servers run CPU-bound C++ apps you wrote? If not, don't bother eliminating class hierarchies, optimize your core logic instead.

Do your webapps spend most time waiting on database and microservices? See if you can eliminate or cache those, the wins are going to be much bigger than rewriting it in Rust.

Is your GraphQL compiler too slow? Unless you are FAANG with hundreds of thosands of developers and hundreds of people to spare, you will get much more bang-for-the-buck with some smart caching or just getting a bigger machine for CI jobs.

According to levels.fyi, the average senior SW engineer in San Francisco is $312k/year. With overhead, the actual cost is likely $500k/yer or so. There is _a lot_ of servers you can buy for that money before you can justify maintaining your own custom version of the existing software solution.