In a nutshell my belief is that Apple will have a few quarters of declining revenue and that is going to torpedo their market cap.
* Apple is 100% reliant on selling new hardware, whatever push into subscriptions they are trying to do is going to fail or take ages to catch on. The reason for this is that Apple sells primarily to consumers, so the subscription needs to be a consumer subscription. What could that be? $30 a month for Apple Music + Apple TV + Apple Fitness + Apple Photo Storage? I suppose this is possible but I don't see people switching away from Netflix, HBO, or Spotify so easily. Spotify is likely the most at risk. This strategy is possible but I think the timeframe is going to take longer than people expect.
* We are close to reaching the end of phone innovation. The things on the horizon that could possibly drive continued revenue are: smaller iPhones, folding iPhones, planned obsolescence, and from the software side a merge of iOS/iPad OS such that you can use your iPhone 15sMAX-SE with a USB-c monitor as your computer. This is possible but say in 2021 apple releases a 4" folding to 8" iPhone with USB-C out to an external monitor that would fly off the shelves. However that is basically the logical end-point of smartphones and after that Apple is going to have a hard time selling anything new until AR glasses arrive.
In a nutshell Apple needs a subscription model to sustain this valuation, and without obsolescence essentially being a forced subscription (you rent your 1000 iphone for 4 years and then chuck it as worthless) I'm not sure they will find success.
The planned obsolescence thing is a real revenue driver for Apple though and without some antitrust lawsuit maybe they can maintain that for a long time. My bet is that 2 quarters from now they miss earnings and the valuation blows up.