Presumably they will offer 7 as an option for a while so as to let people choose when they want to upgrade before eventually forcing the upgrade when unpatched security holes in 5.x make it too great a risk to allow it - keeping in mind that 5.5 reaches end of life in July this year and 5.6 only has 12 months of security patch support after that so that 5.x will be officially dead in less than 18 months time.