I think you're understating the impact of discount rates here. Long term interest rates are down ~70 bps from their previous all-time low in 2016 and something

I think you're understating the impact of discount rates here. Long term interest rates are down ~70 bps from their previous all-time low in 2016 and something like 130 bps from their average over the period you're describing. That's a huge change, especially if you expect Apple's profits to come in over a long duration. There's probably been a shift in investor preference in favor of equities too (which means decreases in interest rates are smaller than the decrease in discount rates used for investors' DCF valuation of equities), but I think "mania" is far too strong a word. I think of it more as a result of investors shifting to riskier assets as returns to capital decline in general, in part due to leverage aversion. I mean, how else are you supposed to deploy your capital? Buy the junk bonds that are currently being issued at yields below 3%?