If something inside breaks, you can get it fixed. For free, if it's under warranty. Most people have neither the time nor the technical ability to fix something themselves anyway, so this is a non-issue.
For me, I've had an iMac for 5 years and I upgraded both the RAM and the HDD (to an SSD) myself with no issues. And it shows no signs of slowing down and is better than the day I bought it, and is probably one of the best investments I've made. Hell, at the time, and this is still true, buying a monitor of comparable quality would've cost half what I paid for the entire computer.
For me, I've had an iMac for 5 years and I upgraded both the RAM and the HDD (to an SSD) myself with no issues. And it shows no signs of slowing down and is better than the day I bought it, and is probably one of the best investments I've made. Hell, at the time, and this is still true, buying a monitor of comparable quality would've cost half what I paid for the entire computer.