I've gone and set myself off on a peculiar train of through here
Just out of idle interest, I looked up Neodynium magnets to see what sort of strengths they come in at what cost.......
I honestly think it could be viable ya know.......
Just the first site that Google throws up, so no attempt to find how cheap they could go, but
THESE are rated for 25kg of pull........That is flush on a contact surface though, so let's imagine there's a plastic or aluminium sheet that the top box or pannier sits on........The magnets could be on the underside of the box, or a steel pad could be epoxied to the corners with the magnets remaining on the bike rack
4 of those, one at each corner, and it'd take 100KG of vertical force (let's say 75kg as we've ferked about with the distances a little

)
It would still be removable easily though, by twisting the case on a horizontal plane then lifting it
EDIT....4 of those come out at about 50 quid (and that was first result, so probably available a lot cheaper)
So 50-60 quid including a couple of tubes of Araldite and a few Steel pucks.......and you have an almost invisible but immensely strong quick release top box system
Just don't put your laptop in the topbox
What am I missing out on here.....why isn't this being done?