Evolution then devolution of iMac "longevity:
* CRT: PITA to get into, but a screen so thick you'd have to smackit with a golf-club to nick it.
* dome, G4: Required Torx and Phillips screwdrivers, but far simpler to srvice than CRTs. "Soft" LCD screen could weather light impacts, or dimple or gouge rather than shatter.
* white, G5: loosen (not remove) some Phillips screws, and the entire screen assembly opens as a lid to reveal the motherboard and other components. Ram and drives (and capacitors) all easily replaceable. These were were some of the easiest machines to service since the "beige" tower era, and used full-sized ram sticks.
* white, core duo: a more complicated procedure to get the outer screen off; flimsy plastic ram-extractor levers easy to break, then more screws and some shroud peeling to get the inner screen off. Flimsy ribbon cables to the camera. (iMacs from this point forward use short-stick ram.) The 24" model of this era had the best-sounding stereo speakers ever put in an Apple device.
* blackbacks: the first iMac with outer glass, but it was protected by the aluminum case in which it was inset. Teardown procedure similar to previous model, with added step of needing a suction-cup to pull the glass off.
* silverbacks: The outer glass now goes to the edge of the device (without that resulting in a larger viewing field) for no legitimate reason whatsoever, and the glass was slightly thinner. Several flimsy cables inside were easy to strip, and one notorious 27" version had a monitor socket prone to tearing right off the motherboard if a cockroach farted in the general vicinity. However, everything was still repairable, and all models of both screen-sizes from 2009 to 2011 boasted a very generous four ram slots capable of 32gb of DDR3 (an absurd amount of expansion potential given that the date-of-manufacture OSes ran comfortably in 2gb).
* thinsides (2012-2020): outer glass is even thinner, and glued to the underlying screen-assembly (a "heat-table" is required to separate the two; hair-dryer tricks usually result in a shattered screen); alternatively your friendly neighborhood Apple tech will quote you a whole screen-assembly unit that exceeds the resale value of the machine (and they'll charge you labor to put it in). Getting the screen assembly off requires cutting a felt liner and cleaning off lingering glue. (This marks the first machine by Apple in which a part must be destroyed in order to service it). Ram was allegedly soldered-in on 21.5" models; this claim is untrue, but upgrading requires completely disassembling the entire layer-cake, as the ram is on the backside of the motherboard! (The ram in 27" models was far easier to exchange via an external door.)
* silicon: these iMacs are essentially keyboardless MBA guts mounted vertically on a stand behind a monitor. Drives are soldered in, few if any parts are serviceable by independent techs (to whom Apple now withholds parts), and the models are planned-obsolescence as they'll auto-brick upon drive-failure). Imacs also now require an external power-supply (like a laptop) instead of any old generic black three-prong power cable if necessary to replace the original as in days past. (I can see Apple's ScrewCustomer Dept, irritated by a used market full of intel-era product only needing a $5 wired key/mouse/cable set to be up and running, sat in a huddle brainstorming, "Let's make the power-supply proprietary, and to add insult, we'll put the WiFi in the brick so that nobody will be able to use these iMacs after the corporate offices writing them down after three years send everything to the recyclers in different boxes like they do with macbook chargers, and the power-supplies get lost!")
