Interesting about 70-300. The current 70-300 is a great lens; light weight and reasonable sharp. Wonder what the Canon-gods have in store for us.
This is all due to the same reason: The future high mp ff bodies need sharper lenses, and these will be more costly to produce plus Canon will take a hefty premium.
The current 70-300L/100-400L might be the last sub-€2000 telezooms we have seen, so in the future dslr tele photography might be well out of reach of the average joe.
I'm not convinced this is to replace the 70-300L.
Because it is f/4.5-5.6.
So I think that this design is to replace the 70-300 IS USM (non-L)
And that this lens will come in at around the $700-$800 (maybe more) mark, similar to the change in price from the 28/1.8 to the 28/2.8 IS.
The catch however is that it needs to be at least as good as, if not better than, the optical performance of the 70-300L. Otherwise it will be a non-event and not worth buying due to the Tamron 70-300VC being about similar in terms of optics but much cheaper.