I.e. if you are saying "we feed the cartel, let's not do anything about it, since doing anything will only potentially help later, so we still need to feed the cartel in in the interim" doesn't really stand any argument grounds. I.e. feed the cartel and do nothing is worse than feed the cartel and do what you can to stop that over time.
And their piece of this is pretty big (huge portion of TV market), that's why they in particular should be asked more than others, why they aren't doing their part.
I doubt they would meaningfully save money over investing in DP, and the opportunity cost is greater for them to spend that money on the next "Frame" TV or whatever.
LG, Samsung and Sony are the only actual panel manufacturers and they probably bake those license fees into the panels they sell back to HDMI Forum.