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One mistake was providing 2to3 as a migration path. This required projects to move completely from 2 to 3, for libraries it meant that they had to either stop supporting 2 (which were the majority of their users) or forking and maintaining both versions. The remedy was to make it possible for libraries to be both 2.7 and 3.x compatible, using tools such as six.py and making some changes to 3.x to make it more compatible with 2 (e.g. adding u'' strings).

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The problem with six is that once you decide to use it your entire code base must be written with the lowest common denominator in mind, in this case py2.7. You can't use six.py at the interface boundaries and nice python3 features inside your implementation. Six is only useful for old 2.7 libraries that want to quickly be 3.x compatible and still maintain their 2.7 compatibility, not if you have an old 2.7 application and want to incrementally add 3.x features.

The proper way make a major breaking change like python did is to provide a bridge between the two versions, so you could import 2.7 modules from 3.x applications, and even the other way around. As of today there is still no such thing.

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