They still have enough in the bank to survive several systems flopping. And the 3DS's success certainly helps. The only reason the stocks slid is because of the change in sales forecast that'll result in an operating loss.
If they had as little money as Capcom has in the bank, then we'd have to be worried, but with where they are right now, they'll be fine.
And keep in mind that while the Wii U's fail to succeed as much as they'd thought is part of the operating loss, it's not all of it. Increasing R&D budget, hiring new people, buying new buildings... all that stuff also contributes to it.
Really, the stock slide is because investors are worried. Any company that 's traded and reveals somewhat bad financial news (low sales forecasts and stuff) suffers similar stock slides. It's just how it goes.
Plus that quote about how they need to fail more before they'll change is blatantly false. That article conveniently ignores another quote from Iwata regarding this whole situation that's quite telling.
"The way people use their time, their lifestyles, who they are have changed. If we stay in one place, we will become outdated."
If Iwata was truly as stubborn as people believe, we wouldn't be hearing this now. All the change in budget and stuff is to change, to find ways to fit in. They don't want to be left behind, so now they're figuring out how to keep up. Admittedly they should have already done this, but better late than never, right?