10.5 is already far more secure than any other mainstream OS out there and equal to BSD/Linux. Even 2-3 years after Leo stops getting updates it will still be more secure than some.
The main thing that makes a secure computer in the first place is a discriminating knowledgeable user. If someone is ignorant to proper security in the first place then even the most secure modern OS on earth won't save them.
Obviously I am zealous. My solution is for me and me alone and I recommend everyone's be their own. Pardon me for not letting the industry dictate what I use and actually having a preference that goes beyond performance I don't need.
I am obviously no average user. Apparently you're a little slow at picking things up.
You can only speak for yourself as can anyone else..
Of course you're not an average user, you're quite simply an odd iconoclast who is willing to spend an inordinate amount of money and make all sorts of usability and productivity compromises in order to keep PPC as your primary platform far into the future. And you vision of computer security as being completely up to user behavior is quite flawed, this isn't ten or fifteen years ago when the vast majority of malware relied on user behavior to initiate infection. Most modern malware actively seeks to infect, either scanning the internet for networks with open ports and known security vulnerabilities or malware is embedded into webpages and launches direct injection attacks.
Users running older unsupported versions of Java or older TCP/IP stacks ARE vulnerable, you DRASTICALLY overestimate the security of the platform.
And the needs of the Average user are easy to estimate.
They are security, vendor support, cost-effectiveness, compatibility with work or school, and the ability to get tasks done in a timely manner.
Not being able to use a promising program that one encounters, work requires, or one might want to use because its 'Intel Only', or not being able to get a bug that inhibits the usability of a program fixed because "sorry PPC is no longer supported" is a significant affront to usability as it is, and it will only get worse over time.
The functionality of a system starts to evaporate when the software eco-system that has been supporting it evaporates.
Adobe just shipped its last version of PPC-Flash, which works fine for right now, but what happens when it ships Flash 11 or Flash 12 and the vast majority of the content out there no longer accessible to you because you are told to install an updated plugin that doesn't exist?
What happens when web-content over the next few years becomes an order of magnitude more demanding, rich web applications start to appear and browsing the web on your machine becomes painfully slow?
These are the signs that for the average person, a PPC Mac is no longer going to be suitable as their primary machine, but you'll overlook all of that the way a creationist overlooks the fossil record.