No.
Wrong.
Sorry... nothing personal; I've just seen this type of bs myth perpetuated for long enough. I can't really stand it any longer.
I bought my "Late 2009 27" iMac" in January of 2010. It shipped with Snow Leopard (about four months old, at the time), the middle of the next year I updated it to Lion, the summer after that I updated it to Mountain Lion... the following late fall, I updated it to Mavericks (for the 1st time doing a clean install, rather than an upgrade), I now am running Yosemite.
Now, I will note that when I bought my iMac, I got it BTO (though NOT top end)- it has a quad-core i5, 8gb RAM, & a 512mb video card.
However... my iMac is showing NO signs of age. I do light video editing & DVD authoring regularly, and it (along with some torrenting & my Netflix membership) serves as my television... I watch at least 15 hours of video on it weekly. My son plays Minecraft, Assassin's Creed, and myriad other Steam games on it, as well as using it for all of his school assignments.
If I had to guess, I'd say as it comes up on its fourth birthday, that it MAY be approaching the halfway mark in its lifecycle.
I do NO maintenance on it.. & NO upgrades. As it has now ran five iterations of OSX on it, just swimmingly... and will certainly run at least the next few... I can patently call out your claim of "each version needs twice the RAM" as a baldfaced lie! Lol, were that true- I'd need to have 128gb RAM right now... or at least 64gb, if I hadn't started out with the upgrade to 8gb.
You are a complete alarmist & just plain incorrect.
The only part I can't be sure about is boot times... I suppose you may be right. They may be longer now. I neither know, nor care. I boot my Mac perhaps 3-4 per year, including OS upgrades. Other than when absolutely necessary, I don't shut it off. Why would I??? It is a home computer. Sleep is fine & consumes no power. It's not a PC, so this doesn't cause any issues. Frankly, I cannot imagine paying the slightest amount of attention to boot time, much less pretending like it has any bearing whatsoever on system performance, when booted.