Same here, there are some old G3s around at some clients, they suck. Not because they're MACs, there are many reasons for their suckage. One client is road-testing Mac minis...cute for sure but I think the novelty of cuteness may wear off, then a critical analyis of fitness for purpose will begin.
Mac's are upgradable, but often (but not always) at higher cost, increased difficulty, and more limited software availability. Also, the physical design of some models, being led more by form than function often reduces expansion potential. The MAC Mini is a case in point, though to be fair the same can be said of any SFF or customised device of any heritage.
Apple stopped selling G3s about, oh, 4 years ago.
Your client is only buying Minis because they're cute? I can't imagine a more idiotic reason for buying a computer, however, if that is the only reason, I can see why they're buying Macs, I've yet to see an attractive small desk top PC.
The average desktop computer has a lifespan of 4 to 5 years. While upgrading can prolong the life of a computer and save the planet to boot, with Firewire and USB2, peripherals can easily be added on. Memory is easily added and hard drives are easily changed on any Mac. You make it sound as though Macs are impossible to upgrade.
The Mac Mini, is not representative of Apple's entire line. It is a niche product designed to serve as an low end Mac. It serves its purpose just fine. Since it takes up such a small amount of space and uses so little material, it appeals to those who don't want some beige box cluttering up their space.
Design is important to Apple and the world is filled with poorly designed products. I would rather spend a little more on a product that not only looks appealing but is well thought out and addresses real life issues.
The Mag Safe power adapter, the responsive keyboards, the anti shock hard drives, the two finger pad which scrolls vertically, horizontally and diagonally are all extremely useful to me. When you spend money on crappily designed products you're always going to forego functionality.