First off I want to say that I think that Macs are fine computers, most Mac owners are very pleased with their purchase.
The Apple model is different than the Microsoft model so you can't just extract one facet of a model and hold it against the whole other model. Apple is mostly a hardware company while Microsoft is mostly a software company. The respective models have served both companies well, Apple is the largest technology company in the world and with annual revenues of $165B it ranks at number 15 on Forbes list of biggest public companies, Microsoft is at number 41 with revenues of $75B.
Now lets debunk the "forever free Mac OS upgrades" myth. For starters I ask that you find a published Mac OS lifecycle support policy from Apple, if you find such a policy I will then ask you lead an expedition to find yeti, I suspect you will have more luck finding the later. Microsoft has a clearly stated published policy to which they can be held accountable, Apple on the other hand has never publicly stated anything about OS support so they can't be held accountable to anything, the old motto of "if we say nothing we can't be accused of lying" holds them accountable to nothing! The word out there is that there is an n-2 support policy but that is just words based on Apple's past practice and while it does seem to follow practice Apple has never commented on the supposed policy, let alone confirm it.
Now lets look at the facts:
1- To upgrade to the latest Mac OS x v10.9 "Mavericks" you must have at least v10.6 "Snow Leopard" installed, if you do not have at least Snow Leopard you must purchase the $20 upgrade, not very expensive but still not free.
2- If you would have bought a Mac desktop in 2001 when Windows XP was released you would have bought a Power Mac G4 with OS X v10.1 "Puma" installed. The Power Mac G4 cannot be upgraded to Mavericks (hardware not supported).
3- In early 2006 you would have bought a Power Mac G5 with OS X v10.4 "Tiger" installed. The Power Mac G5 cannot be upgraded to Mavericks (hardware not supported).
3- Between August 2006 and April 2007 you would have bought a Mac Pro Woodcrest with OS X v10.4 Tiger installed. The Mac Pro Woodcrest cannot be upgraded to Mavericks (hardware not supported).
4- Between April 2007 and January 8, 2008 you would have bought a Mac Pro Clovertown with either OS X v10.4 "Tiger" or v10.5 "Leopard" installed. The Mac Pro Clovertown cannot be upgraded to Mavericks (hardware not supported).
5- After January 8, 2008 you would have bought a Mac Pro Harpertown or later model, these Mac Pros can be ugraded to Maverick (hardware supported).
6- If you bought your Mac Pro Harpertown before August 28, 2009 it shipped with OS X v10.5 Leopard so you will still have to pay $20 to upgrade to the minimum Snow Leopard requirement before you can upgrade to Mavericks.
7- OS X v10.5 Leopard is no longer supported, support ended when v10.7 Lion shipped in July 2011, the last security update for Leopard was in June 2011. Leopard was released in October 2007, figure it out support for Leopard lasted less than four years, and if you were one of the unfortunate who bought an expensive Mac Pro Clovertown in early January 2008 the whole expensive thing was supported for a grand total of about 42 months!
8- If you are running Snow Leopard on any Mac before the Mac Pro Harpertownd and following Apple's n-2 OS support practice it's a pretty safe bet that support for Snow Leopard has ended and that you are at the end of the line with your pre Harpertown Mac... of course no one from Apple will either confirm or deny this because it would hurt their "hardware model" if everyone knew the real truth behind the "free" OS upgrades.
You see, Mac OS upgrades are free but you have to buy a $3,000 Mac every 4-6 years to run the upgrade. In reality Mac operating system versions are rarely supported for more than four years and if your Mac cannot handle the newest version well tough bananas, either run it without support or go buy a new Mac! On the other hand for about 13 years and as long as the hardware suppliers supplied drivers you could have installed Windows XP on just about any computer, even on an old Mac no longer fit for Snow Leopard and Microsoft would have supported it!
John