I think Williams could have kept going and survived if their mission was pinball, but bean counters were in charge and they went where the money was. It's probably good for Stern they did or it may not have been the survivor without having the market to themselves.
I do like Stern games and own a bunch, but I do miss some of the charm and humor of WPC games. Many seem to have caught an intrinsic magic missing today. Newer Stern's can be just as fun to play, but in a different way. From my perspective, Stern is getting there in a "cheaper" way, cashing in on licensed themes, while Williams could get the job done without. In that way, I still tend to think of Stern as second fiddle. Let's see if Stern can catch the magic with an unlicensed theme. We won't have to wait much longer now to find out, and I hope it's a smash, as none of the music themes have caught my full attention. I did however enjoy AC/DC and look forward to try IMDN.
I'm also not totally happy with some of the design evolution at Stern. It's clear software plays a much bigger roll these days, but a lot of the resource now has to go to the LCD display. I think this has hurt pinball evolution in some ways: personally, I'd like to see more programming attention in other areas like the light show. For a while we had full-rgb playfields that were well-utilized, as in Star Trek premium/LE, along with full power flashers. In some sense, I think the SAM platform was the pinnacle. I haven't looked closely at recent releases (maybe it's getting better?), but early Spike machines had some pretty dimly lit inserts with cheap pcb LEDs and flashers without much punch. This could also be a result of cost cutting with the power supply and not having the same versatility there.