This isn't about ageism at all: it's about something much bigger.
This poorly conceived management decision has just become something else for politicians to kick the BBC over - and, let's face it, the BBC has absolutely no political allies now. The Tories have always been baying for their blood and Labour is now battling with them over plans to reduce the Licence Fee.
So something the Daily Mail can get itself in a lather about (:yawn

is, of course, fuel to the Tories and the ridiculously short-sighted Harman. Well done, BBC - your own goal just keeps on scoring.
Monkseal is quite right: Arlene's presence on other television shows would be governed by the SCD team.
They are ultimately responsible for the mess last year descended into (after all, the judges report
to them) - and they have elected to start SCD7 with a major PR disaster. I am personally fine with waiting to see how Alesha fares; I am less fine with watching one of my favourite programmes being run by a bunch of half-wits. Lessons should have been learned from last year ... you would think?