On the face of it, yes it would seem to make commercial sense.

But with emphasis on the word seem ......

There is a fine balance between getting enough people to book J class tickets at full revenue (ie well in advance of departure date); and creating an impression (and I mean in general ..... not just in relation to any one specific flight) that heavily discounted seats will be available at last minute/check-in - a scenario which would then encourage a certain proportion of pax who might normally pay full fare in J to instead book Y and then sit tight & hope for some sort of upgrade 'deal'

I'm certainly not saying that the boffins who work in the weird & wonderful world of yield management always get the mix 100% right. But there is obviously pressure on them to do so most of the time, simply because the alternative spells doom for an airline.