In my opinion, asking for "removing unused features" is one of the consequences of full rewrites. Also, a reason for having to maintain "dual systems" for years

In my opinion, asking for "removing unused features" is one of the consequences of full rewrites. Also, a reason for having to maintain "dual systems" for years. It is not infrequent having "dual systems" and having to deprecate the "new", just because after e.g. 10 years you can not get rid of the old, or even worse: the "new" performing worse than the old (e.g. old simple centralized system working much faster on new hardware vs "new" pseudo decentralized mess that can not scale as good because inter-node communication overhead). BTW, every time I read Martin Fowler quoted in a post as full rewrite justification for the "faith jump", I get scared. I don't know why the poster didn't implemented the new API, using it for the new features, and eventually, rewrite old API commands (one by one, not all at once) calling the new API, so in the end, new API would remain, and old API would be just some wrapped calls to the new API.