I'd never heard of a 2-year upgrade cycle. According to this, 3 years is/was more common. If it's gone in any direction, I'd guess it has increased
a bit, since then.
https://www.nextplatform.com/2015/11/05/three-is-the-magic-number-for-hardware-cycles/
InvalidError :
Kewlx25 :
Even when spending several thousand dollars on a server, it has become cost effective to replace servers every 2 years because of the energy savings.
If you build a server that consumes 500W, it will cost you ~$500/year to power and cool
That's some pretty cheap electricity! At $0.11/kWh, it's cheaper than my residential rate. And that's not even including cooling. Where did you get those numbers?
BTW, they do need to add capacity, over time. And they have finite space in which to do it. Just another factor to consider, but the above article goes into considerably more depth (if you care about that sort of thing).