Backward compatibility: why ???
Why should openGL 2.0 be backward compatible with openGL 1.3 ? Vendors can provide both drivers, as 1.3 ones won't be maintained anymore...
Every OS I know can manage shared libraries versions.
Re: Backward compatibility: why ???
Presumably so that ISVs can migrate large codebases to the new API smoothly - just because you've got both 1.3 and 2.0 drivers installed doesn't mean you can load 'em both up and have them share a rendering context.
Professional graphics systems like CAD tend to be *huge*. Game engines look like toys by comparison. Forcing people to port all-at-once or not at all would severly hamper adoption of the new API.