GW has "modernized" their rules what, 12 times now? A lot of those revisions are what pushed them to the brink of bankruptcy in the first place.
I suppose it's a question of what you consider "modernization" and how you approach the existing community. The trick, one not everyone can pull off, is to create a version of the game that brings in gamers accustomed to modern streamlined rulesets, while not alienating your existing fan base. I will grant you that is not an easy trick to pull off, filled with hazards that could destroy a franchise and even if done successfully may not work simply for market/economic reasons.
That said, business is about taking risks, as the case is with Battletech, in 35 years not much has changed in the game and while that probably appeals to fans (even I find it rather quaint and comfortable in a way), it does not, nor will ever lead to any growth for the game. Growth, in business in general, but very specifically in the table top gaming business comes exclusively from reinvention, from new directions and expansion. You can't expect to put out yet another box set with the same rules you have had for 35 years and expect a sudden influx of new players. As was already pointed out, the vast majority of box set was sucked up by existing fans hungry for miniatures for a game they already love.
In any case someone asked the question which I found rather interesting about how one might go about adjusting the existing rule set. My friend and I briefly talked about this after our 6 hour session with the game and there was one immediate red flag for both of us as far as the game mechanics were concerned. Attrition.
I think the attrition in the game was really the thing that caught our attention as a core factor that extends the game beyond reason and the inability to affect it in any way tactically. These mechs are really sturdy and even if they are just one hex away from each other standing still, it takes many rounds before someone goes down, even if all shots hit. I mean at the end of round 10 in our game we had two mechs basically at melee range shooting at each other and it took another 5 rounds of shooting before someone went down and there was ZERO one could do to affect that except to get lucky with a location hit in the head or the legs.
The game also suffers from bookkeeping problems. With four mechs on each side your managing 8 mechs and it was really hard to keep straight for example the shooting declaration phase. I mean you had to track which weapons were fired by which mechs at which opposing mech for an entire phase. Almost every round of shooting we couldn't remember which mech targeted which opponent with which weapons. Its a really terrible way to handle that phase.
In general though we found it really difficult to remember all of the modifiers that affected any given action in the game, especially once the mechs suffered critical damage.
I would definitely address the general attrition of the game and the bookkeeping of the games modifiers, and in particular the declaration phase (I would rework that entirely). In fact in our game about halfway through we realized that, if you just declare and shoot at the same time, the game was 90% faster and easier to manage and their was virtually nothing lost tactically.
Those are just some of our early observation, I think a lot more could be said about how the phases and bookkeeping is managed, but those were like the main things that really dragged what really should be a 1-2 hour game at absolute maximum into a 4-6 hour game.