I must admit I do sometimes wonder how I would cope if Google decided to pull the plug on my account. For me, I would find life without Google (especially GMail, Google Docs, Google Reader and Google Maps) very hard indeed. It's happened to Chris Brogan. Read his post When Google Owns You | chrisbrogan.com.
Finding a replacement which doesn't suck would be hard right now. I just don't really like Yahoo or Microsoft Live much (though I have accounts on both), and even with a replacement service I'd still not have access to the giant heap of old email and documents which Google does such a great job of storing and searching for me. And, despite other peoples' suspicions that Google no longer lives according to its founding principle ("Don't be evil"), I instinctively trust Google much, much more than Microsoft or Yahoo.
All this great Google stuff is completely free, so it's very difficult for any of us to complain if it does get withdrawn. I don't for one moment think that Google is likely to do that without good reason, but it puts all of us in a position of having little or no leverage. Personally, I'd be more than happy to pay a nominal amount for my Google services (by which I mean almost everything except search), to ensure continuity of access, security of my data and a contract which means I'm owed the services. How much? I don't know - what about 30 USD a year? That's unlikely to upset anyone who really wants or needs the service, but if enough people paid it could provide Google with a ton of additional money they could use to invest in making those services even better.
Will Google ever transition to a paid model? The Picasa photo service has a free and paid service already, probably because Flickr and others do this as well as to cover storage costs. But for the rest of Google, I'm not sure. You can imagine how administrative overhead might soar with paid accounts: not only would they be obliged to keep track of everyone's identity, account and payments, but armed with a contract, the kind of people who are intolerant of even occasional outages would feel entitled to complain loudly. I wonder if the folks at Google have 'done the math' and decided that any additional revenue would be eaten up by this sort of thing?