Over at EmergentChaos, Adam asked what happens when "the Snail" gets 10x worse? I need several cups of coffee to work that one out! My first impressions were that ... well, it gets worse, dunnit! which is just an excuse for not thinking about the question.
OK, so gallons of coffee and a week later, what is the natural break on the shift in the security marketplace? This is a systems theory (or "systemics" as it is known) question. Hereafter follows a rant on where it might go.
(Unfortunately, it's a bit shambolic. Sorry about that.)
A lot of ordinary users (right now) are investigating ways to limit their involvement with Windows due to repeated disasters with their PCs. This is the the first limiting factor on the damage: as people stop using PCs on a casual basis, they switch to using them on a "must use" basis.
(Downloading Firefox is the easy fix and I'll say no more about it.) Some of those - retail users - will switch to Macs, and we can guess that Mac might well double its market share over the next couple of years. A lot of others - development users and poorer/developing countries - will switch to the open source Unix alternates like Linux/BSD. So those guys will have a few good years of steady growth too.
Microsoft will withdraw from the weaker marketplaces. So we have already seen them pull out of supporting older versions, and we will see them back off from trying to fight Firefox too hard (they can always win that back later on). But it will maintain its core. It will fight tooth and nail to protect two things: the Office products, and the basic windows platform.
To do that, the bottom line is that they probably need to rewrite large chunks of their stuff. Hence the need to withdraw from marginal areas in order to concentrate on protecting that which is core, so as to concentrate efforts. So we'll see a period characterised by no growth or negative growth by Microsoft, during which the alternates will reach a stable significant percentage. But, Microsoft will come back, and this time with a more secure platform. My guess is that it will take them 2 years, but that's because everything of that size takes that long.
(Note that this negative market growth will be accompanied by an increase in revenues for Microsoft as companies are forced to upgrade to the latest releases in order to maintain some semblance of security. This is the perversity known as the cash cow: as the life cycle ends, the cash goes up.)
I'd go out on a limb here and predict that in 2 years, Microsoft will still control about half of the desk top market, down from about 90% today.
There are alternates outside the "PC" mold. More people will move to PDAs/cellular/mobile phones for smaller apps like contact and communications. Pushing this move also is the effect we've all wondered about for a decade now: spam. As spam grows and grows, email becomes worse and worse. Already there is a generation of Internet users that simply do not use email: the teenagers. They are chat users and phone users.
It's no longer the grannies who don't use email, it is now the middle aged tech groupies (us) who are feeling more and more isolated. Email is dying. Or, at least, it is going the way of the telegram, that slow clunky way in which we send rare messages like birthday, wedding and funderal notices. People who sell email-based product rarely agree with this, but I see it on every wall that has writing on it  .
But, I hear you say, chat and phones are also subject to all of the same attacks that are going to do Microsoft and the Internet so much damage! Yes, it's true, they are subject to those attacks, but they are not going to be damaged in the same way. There are two reasons for this.
Chat users are much much more comfortable with many many identities. In the world of instant messaging, Nyms are king and queen and all the other members of the royal family at the same time. The same goes for the mobile phone world; there has been a seismic shift in that world over to prepaid billing, which also means that an identity that is duff or a phone that is duff can simply be disposed of, and a new one set up. Some people I know go through phones and SIMs on a monthly basis.
Further, unlike email, there are multiple competing systems for both the phone platform and the IM platform, so we have a competition of technologies. We never had that in email, because we had one standard and nobody really cared to compete; but this time, as hackers hit, different technologies can experiment with different solutions to the cracks in different ways. The one that wins will attract a few percentage points of market share until the solution is copied. So the result of this is that the much lauded standardisation of email and the lack of competition in its basic technical operability is one of the things that will eventually kill it off.
In summary so far; email is dying, chat is king, queen, and anyone you want to be, and your mobile/cellular is your pre-paid primary communications and management device.
What else? Well, those who want email will have to pay *more* for it, because they will be the shrinking few who consume all the bandwidth with their spam. Also, the p2p space will save us from the identity crisis by inventing the next wave of commerce based on the nym. Which means that we can write off the Hollywood block buster for now.
Shambolic, isn't it!
 "Scammers Exploit DomainKeys Anti-phishing Weapon"
 "Will 2005 be the year of the unanswered e-mail message?"