Give the D&D IP rights to a holding company that has no other business model and I think we'd see some action in this regard.
Probably not even then. Deep pockets make the tactic possible, but don't mean you'll do it. There are two basic times it is worthwhile to try such a lawsuit:
1)The damages the court will award you are substantially higher than your legal fees and other costs associated with the suit (note, the time your execs spend on it counts!).
2)If you win, you stand to reap more extra money out of the IP than you paid in legal fees. (This may be "in the long haul").
The current 3pp folks are, compared to WotC/Hasbro, small fry with small sales. Trying for (1) is trying to squeeze blood from a stone. And, when WotC itself is no longer getting sales from prior editions, (2) is unlikely.
Basically, one of the 3pps has to make it big-time before there's an economic advantage to such a suit.