[joshuamorton]

There's always been a very strong one version policy, multiple versions are usually only allowed to coexist for weeks or months, and are usually visibility restricted.

This prevents situations where "Gmail" ends up bundling 4 different, mildly incompatible versions of MySQL or whatever, and the aggravation that would cause. Or worse, in c++ you get ODR violations due to a function being used from two versions of the same library.