The Mac Performa 560 also stuck around forever, and the LC/PowerMac 5260, and the 5400. Apple is no stranger to leaving machines in production and on sale forever.
Part of this was that they weren't particularly good at actually clearing their retail channels, which is part of why you had situations where, say, the PowerBook 1400 was still relatively available in 1998, and where the UMAX C500 and S900 took until at least early 1999 to clear resellers.
This was a specific weakness they had identified as early as either their annual report in 1996 or 1997, and given his resume, it's almost certainly one of the reasons why Tim Cook was hired in 1998.
Re modern product cycles: Apple keeps things around forever today, too. The MD101LL/A (2013 13" MacBook Pro w/ i5 2.3 4/500) didn't quite outlast the Mac Plus, but the Mac Pro 6,1 has, and it wouldn't surprise me if ultimately the 2015 MacBook Air (which is still on sale in 2019, despite a 2018 refresh being available) does, and the 2014 Mac mini might have. Plus, old flagships is typically one of Apple's main source for "more affordable" iPhones, and you have situations like the iPad mini which just got refreshed last week after around four years.
The
other thing to consider when looking at Mac specs and introduction/discontinue dates is often specific sub-models will have different dates. For example, the
LC 475 might have been discontinued in 1994, but the
Performa 475, which was an entirely different machine on Apple's books, and which had an entirely different retail supply chain and sales environment, might have been sold until 1996. (spoiler alert: I looked, and that's, in fact, the difference, which I didn't actually know up front when I started writing this.)
Relative to their relative the Quadra 605, the LC and Performa 475 did share case components, and people often ended up with large piles of them, so if you have a machine that has LC475 label on it, but a non-475 build date, it might actually have started life as a Performa and had its top case swapped at some point, for whatever reason. That is also common with Quadra/Centris 610 and 650 machines, the II/x/fx, and there are a few other situations (630/6200/6300/5200/5300, 5400/5500/6400/6500), but most of those are more obvious because the graphics or CPUs change from different models, a /180 will always be an x400 and a /225 will always be a 6500, for example.