The three-letter word
The domain names are used for global identification of resources and navigation. Since they have to be typed only once, three extra characters don't make a significant difference. For machine-generated content, there is zero difference: after the site is bookmarked, it doesn't matter how long its URL is. There will be no perceivable loss in productivity if the address is slightly lengthened. (This is especially true in the context of .xxx domain.)
There is no point in abolishing .com today, but there is also absolutely no good technical reason to add new generic top level domains. Introducing .xxx will not make filtering adult content any easier, since adult sites will continue to operate under .com, with new ones springing up daily. What it will do is create a windfall of profits for registrars as another land grab starts—and not surprisingly it's the registrars who lobby ICANN.
Real progress on restricting children's access to adult content can be made if robust age verification becomes part of the protocol, which requires solving the Identity paradox. As for battles over the root zone, they can be resolved by simply freezing it in the current state, removing unnecessary central control and thereby starting to address the Responsibility paradox.