It's more like a built in limit so that Apple can declare, in 5 years time, that the current macOS of the day requires a T1/T2 CPU (ie as old as the 2016 MacBook Pro but more relevantly, they can say it requires a 2020 iMac).
They will then support the last working macOS that works on non T1/T2 equipped Macs for two further years if you take their current custom into consideration so in effect directly supporting the 2019 iMac until 2024/2025 before ending security patches in 2026/2027.
It will effectively end Hackintosh.
The other thing that T2 CPU in an iMac could do is allow Apple to use F series Comet Lake S CPUs - that is, CPUs that have a failed GPU, because the iMacs would have discrete GPUs.
It's the same thing that allows Apple to use Xeon W CPUs in the iMac Pro - no iGPU but Quicksync is hived off to the T2 which also works as the storage controller and Secure Enclave.
Intel would have a ready supply of these as Apple would no longer need Quicksync and would switch to the cheaper parts to help with the bottom line. Assuming they were still wanting to stick with desktop CPUs in the iMac line.
One other point comes to mind here. If Apple actually went with the Comet Lake H series CPUs in a future iMac they would retain the iGPU as Intel don't offer F series mobile parts.
The point here is that Apple save power in MacBook Pro 15" and 16" variants by running with the iGPU only until something in the system demands more computing grunt from the dGPU. It then fires up. Now, that would be a method of reducing overall system heat and noise in a thinner iMac - if a user who isn't using additional displays and is only browsing Safari. The dGPU goes to sleep and you have a quieter system.
On the other hand, if you're firing up Final Cut Pro, or Photoshop, or a game(!) the dGPU springs into action and the fans can start thinking about spinning up if the user gets intense.
And
Pro Mode is the other side of that coin - if you're caning the CPU as well you can turn up the fans to favour performance over quiet.
So in effect we have a MacBook Pro on the desktop...