Oh boy where to start.
Flash Player is not built into OS X. In fact, you have to install it and update it regularly. This is assuming that you don't use Chrome as your browser.
Why, on earth, are you defending Flash? You're literally going to argue that you shouldn't have to install ClickToFlash (pretty sure almost everyone at least uses ClickToFlash, if not then Adblock) and would rather defend Flash based advertisements and other poorly generated Flash content as a part of your more than likely made up 20% statistic? Flash is generally UNWANTED by most. I realize in some cases there is no choice but what are you ACTUALLY defending?
There is nothing that flash based advertisements, clutter, and terrible optimization has to offer you as a part of maximizing your user experience. Sorry your Facebook games aren't playing smoothly?
You're ignorant if you think Flash is a problem free technology and runs fine on "10 year old computers." It can, but you're making ridiculous generalizations while defending who knows what.
Just because a technology is inefficient doesn't change the fact that it's still widely used, and to suggest someone not use it is absurd. I'm not defending flash, I'm just saying it's everywhere, and to just say "disable it" in order to make the computer experience more in line with what's acceptable, is frankly, unacceptable.
Again, why should I have to go back in time in order to make the computer usable? In almost every post where someone complains about performance on these forums, the common replies are "disable OS X visual items x, y, and z" and then second most common is "block flash\disable flash" ... it's absurd to suggest someone shut off OS elements and industry-standard technologies in order to make their computer's performance acceptable.