people will continue using Python 2.7 for a long time no matter what the deadlines are. there are lots of sites running under Python 2.4 many years after its EOL.
anyway, I share your concerns and looking at the time it takes to release (30 months between Plone 4.3 and Plone 5; around 18 months between Plone 5 and Plone 5.1) I think it would be better to focus on the urgent things for Plone 6.0: Python 3 support and fixing the resource registries story. IMO, all other enhancements must have a lower priority.
what scares me is that seems nobody wants to talk about the resource registries at all: everybody just ignores it when roadmaps are drawn and nobody answers the questions:
also, porting add-ons to Python 3 is a strategic issue that has to be taken seriously; makes no sense to have Plone 6 running on Python 3 with no add-ons for it; let's not repeat the story of Plone 5.