You can (and need) to do both. And OpenBSD does. LibreSSL as one example removed a huge amount of dead/spaghetti/obsolete code from OpenSSL. And they are removing old/obsolete features all the time. Do you use OpenBSD? Do you read the release notes?
Have you used OpenBSD? You're telling them they should be doing something, that is already basically their mission statement.
Maybe OpenBSD itself is better at sticking to these principles than OpenSSH. I haven't used (experimented with) it for ~5 years but read about various updates every so often.
> Normal OpenSSH development produces a very small, secure, and easy to maintain version for the OpenBSD project. The OpenSSH Portability Team takes that pure version and adds portability code so that OpenSSH can run on many other operating systems.
Unless you actually run OpenBSD, what you think is "OpenSSH" is in fact "OpenSSH Portable Release". These are very different things.
There's plenty of flaws with any project, but OpenBSD is pretty well known for doing exactly the thing you're claiming they don't do.