Apple's main website no longer links to that old 21.5 inch iMac sales page. They discontinued 2 of the 4 prior storage options weeks ago. Leading up to the event this week, rumor sites were reporting many Apple retail stores had completely run out of stock on 21.5 inch iMacs.
Pretty sure Apple will not "keep selling that for at least the next decade".
As for what could be considered "pure profit", Apple's M1 chip is said to be about 120 square mm, which is about half size of Intel's latest chips. That half-cost chip comes with an integrated GPU that rivals the lower range of discrete graphics cards. The whole thing runs on only 15 watts, so they also save on power supply, voltage conversion and cooling parts. The whole computer fits onto a tiny circuit board, also lowering cost when manufactured at high volume.
Even at Apple's premium prices these new iMacs are going to deliver a great value, and at a much lower cost to Apple.