How many PDFs are purchased(and in that vein, how much money WotC makes off of them) compared to how many are pirated illegally. If pirated copies are significantly more common than legitimately purchased ones, I don't see where WotC had any real choice here. The issue is competition. They essentially are in competition with free pirated copies of themselves. If the ratio is really bad, they are in an untenable position.
Yes, we all acknowledge piracy is rampant, and it hurts publishers, retailers and legitimate consumers like. But it is unfortunately, a fact of modern life.
I certainly don't fault WotC for pursuing litigation. BUT, with this new stance, they're currently not even giving themselves a CHANCE to compete in the growing PDF market.
For example, I have been buying older edition PDF's (mostly Planescape, DarkSun and Ravenloft stuff I wasn't able to get when they were in print); I only need a few items to complete my collections, but now that seems like an impossibility to do legally for the time being...
That said, I'd dispute that it's more than just numbers at stake.
Beyond the sheer numbers, other factors WotC should consider include:
1) Fan backlash, including alienation of the "PDF-only" gamers, a growing market.
2) Retailer backlash. How is Paizo/DTRPG, etc., supposed to react to this quite-sudden 180 degree shift in policies?
3) Public perception (this ties in to to #1 above.)
4) Company reputation- And this is something that was ALREADY, it seems to me, on shaky ground (IE: The Dragon/Dungeon debacle from a few years ago, the GSL fiasoco, and the DDI still being a hot mess almost a YEAR after 4E debuted..)