They could have looked at ground speed and it would have helped.
So if you lost IAS and then you scanned GS and see it drop from >400kts to <50kts you would not worry?
Trouble is you (and others) are looking at this with the benefit of hindsight, aka "Monday morning quarter backing".
These guys were on the edge or beyond it and they had very little time to decipher WTF was going on. Nowhere, to my knowledge, is keeping the Groundspeed, even if displayed, a requirement of a basic instrument scan. I'm sure, if the AF447 crew had unlimited time then they may well have noticed the G/S and made the connection.
I'll make the point I made a while back: unless you have experience of an aircraft coming "unglued" around you have no idea how idea how difficult it can be to comprehend (a) how little time you've got left and (b) how bad things have got. I suspect if we could
listen to the CVR that would be very clear.