Liking Vilas's comment!
Anyhow, if you base the decel call on the autobrake light, then when you are landing on a contaminated runway, you may end up doing the wrong thing as that light may, or may not come on even when the autobrake is functioning correctly (See FCOM)
So by basing it on speed trend and expected deceleration rate you get a 'professional judgement' whether things are progressing as expected... And remember, both pilots feet are fitted with brake pedals so if it isn't and they aren't, you should!