Apple would never give a "solid commitment" to support X of years because that would draw out the transition.
Sure, new Intel iMacs will come out this year. Because people still need computers today to do work based on x86 programs. But that doesn't mean a $2,000 iMac will slow down the entire transition.
You seem to be conflating the transition and support. They can complete the transition of their entire Mac lineup to their own silicon and still provide support for the then-no-longer-produced Intel Macs for "years to come" (which again, that phrase in the normal use of language is not literal and implies more than 2 years, especially when said with such emphasis as Tim did).