Possible, but not sure it would really have "incompatible" code. If you had the full toolchain (as today), you could build anything. The "light" version would have limitations (likely lacking Mac/iOS developer abilities), but you could always upgrade if you wanted to remove your limitations.
Perhaps it would be more broken into modules vs. multiple versions. So, "light" would be the minimum module. You could add on modules as-needed: Apple Platform support, C interop, TensorFlow, WASM. Today, the toolchain gives you Base + Apple Platforms + C, etc. .. but not TensorFlow (currently a unique toolchain). In this modular approach, you add on piece parts as needed.
Of course, I don't know if this is actually possible or practical.
Yah... smaller download or rather less disk usage. A hesitant developer just dipping their toes in the water may decide 1.6GB is more space than they care to take on their drive. Vs. Rust, where < 200MB is relatively trivial. Ultimately, it's prob not going to be a deal-breaker for most... but could be a hurdle... so the slimmer we get, the lower the hurdle. In the end, you're right... the MOST important thing is an easy installation experience.
I think 1.6GB wouldn't be as noticeable if you downloaded a small toolchain manager ("swiftup" or whatever it's called) and it automated downloading and installing the toolchain for you. It all happens behind the scenes vs. downloading a large installer.