No it will not predict the future direction of Bitcoin or alts.
Sometimes Bitcoin is going up and alts are staying flat or going down.
Sometimes Bitcoin is flat and alts are going up or down.
Sometimes Bitcoin is going down and alts are also going down.
Right now I think many are way too over invested in alts and hence the dominance index keeps rising every few days.
However the settlement can be different next week or next month.
I agree with this point of view and, as many have already noted, the altcoins are often bought and sold using bitcoin or ethereum. This fact alone links their value, but not always in the same direction.