What do you think people who switch from Windows to Mac have to do? Think about applications and hardware that won't work in OS X. All that doesn't stop them from switching.
...but who knows how many more would switch without that obstacle? What do you think has kept Macs largely out of business use? I've used Macs on and off for ever, but I only switched over to Mac as my primary machine when they moved to Intel and Bootcamp/Parallels/VMWare became available.
And as with any technology you'll soon reach a point where it is outdated.
Except... the days when your top-of-the-range computer would be hopelessly outclassed 18 months later have gone. Back in the good old days, when you upgraded your 2-year-old computer, you ended up with something twice as fast, with twice as much RAM and twice as much HD (or even more if you look back to the jump from 8 to 32 bits) that could take on new applications that were impossible on your old wreck. There was a really strong incentive to spend money on an upgrade and upgrade peripherals, buy adapters as needed.
Now, looking at the case in point - upgrading from an early 2011 17" MBP to the current 15" rMBP does not[/n] bring you double the speed, double the RAM, double the storage, double the screen resolution. Yes its faster, but the 'dramatic' speed up comes from using SSD, which can easily be added to the MBP. RAM is the same max 16G, and you can easily end up with less storage unless you spring for the (v. expensive) 1TB SSD (you can put a 1TB SSD in your MBP for less). Even the screen isn't a clear win c.f. the 17" MBP (you're trading sharpness for physical size).
Sure - I'd like a rMBP, for the extra portability (although the 17" MBP is portable enough to shuttle between desks) and the ability to run a second external display... and I'm sure it would be a bit faster - but my MBP isn't exactly slow.
If it breaks you need to shell out a lot of money because usually you'll have to replace nearly everything in the chain.
...again, true in the past when a 2+ year old machine was so outclassed it was not worth spending money on to get fixed. Yeah, I'd probably use the excuse to by a shiny new Mac if my MBP died, but if money was an issue it would certainly be worth looking in to getting it fixed or buying a secondhand replacement.
The problem to some is that they see no reason to upgrade because what they use now is simply working for them. In that case, don't upgrade and stick with what you haveUpgrade when things break but be prepared to shell out quite some money because of outdated tools.
I don't disagree... but that is going to be bad news for Apple and other PC manufacturers who have got used to a world in which people routinely change their computer every 2 years.