Just security updates would be fine with me - how hard can it be? Firefox3 et. al not supporting it anymore, I can live with that as FF2 does the job well, but again security updates would be nice.
Security updates can be costly. Different versions may be different under the hood and require extra work to backport security fixes. Testing is also expensive; a change to security may break another feature.
As long as software vendors come out with software that they can not guarantee to be secure and that is directly exposed to the Internet, I think it is not too much to asked, particularly for popular software or software that is sold bundled with hardware, to provide free security updates for at least eight, preferably ten years.
10 years is a long time in the computer world. Over that stretch of time, computers become 100 times as powerful. Is a Pentium II really a viable computer today? Many people upgrade for new features or because their old computer died.
Also, it's not feasible to support a 10 year old OS. How many people are still running Windows 98, as their sole OS or their primary OS? How many people are running hardware intended for Windows 98 (e.g., Pentium II)? How many manufacturers are making hardware that is compatible with 10 year old hardware? How many are still making drivers for Windows 98? 10 years after the release of an OS, it's ecosystem is pretty much dead and anyone supporting a 10 year old operating system is pretty much chucking money down a hole. The same goes with most software. Would a secure version of Netscape 3 be worth running?
If there were money in supporting software for 10 years, then companies would already be doing it. Some companies will support old OSes for a fee but keeping an old OS up-to-date will require a *lot* of money.
There is far more money is selling you Windows Vista today or Windows 7 tomorrow than there is in providing you with patches for 10 years ago.
Upgrades of OS/X still cost a few hundred US dollars last time I checked.
I can't guarantee prices now but a Leopard upgrade was $99 for a single license and $129 for a 5-pack when I picked it up some time ago.
A better solution for you may be to look into one of the PowerPC Linux distributions. It'll breathe some new life into old, but still good, hardware.