What other kind of incentive should be awarded for boss battles?
The problem with providing incentives that would entice people in top tier gear is that you have to deal with the fact that the resources that top tier geared players actually care about are resources that are restricted on a per day or per week basis. Even if the developers added something like a new option to specifically buy grade 8 material boxes with columi or tionese comms or tionese crystals, you'd still run into the issue of being able to run said content ad infinitum and break the nominal per day and per week limitations that exist for every other source of stabilizers. If you addressed this problem by making the cost of said boxes so high that it wouldn't really be an efficient use of your time to simply farm the bosses to gain the mats to buy the boxes, you'd then have to deal with the problem that people would still want to skip bosses because it's not an efficient use of time. You either screw up the economy and gear progression or you don't address the problem you were attempting to solve.
The only real tweak that I could imagine would be to add a bonus mission to the daily HM FP mission to kill 4 FP bosses or minibosses that provides you with a minor additional benefit (a box containing 2-3 BH comms and a lowish chance at a grade 8 crafting material). Making it a bonus mission attached to a daily mission means that it can't be repeated more than once each day and providing a static benefit that is somewhat useful but not overly so (since BH gear is no longer the bestest gear) along with a chance at something really useful (the grade 8 crafting mat) means that people would actually be willing to do it.