For the same reason, Perl 6 should be called something else as it is not an upgrade of Perl 5
if you give it the same name, but with a higher version number, people are going to expect that the capabilities they had before would still be there, along with the new stuff. If you don't intend to provide a feature, you need to list is as a feature that's specifically being removed (and if you have a very long list of such things, expect to loose a lot of people, each one of these 'removed' or 'unimplemented in the new version' features is a regression)
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