check the table at the bottom of the page.
7D started in 2009Q3. 550D followed in 2010Q1, 600D in 2011Q1 and 650D in 2012Q2. 60D started in 2010Q3, 60Da in 2012Q2. 6 cameras with the same sensor.
life cycle of the xxD was 1.5 years for 10D-20D-30D-40D, 2 years for 50D, and 60D is on the market for 2.5+ years.
as after 7D, the next rebel after 7Dmk2 will get the new sensor. it will probably be the 700D or the 750D.
assuming that the xxD life-cycle continues like this, 70D with the same old sensor will be very outdated in 2014, but we'll have to wait till early 2016 for the 80D (2.5 years from 2013Q2).
do you still think canon will put the same old 18mp sensor in 70D?