Working on mostly server platforms, I had forgotten that IOMMU enablement (and, where relevant, enforcement) was not the default.
Consumer hardware and software is terrifying.
The only thing you're getting by saying "no IOMMU" is "I want any devices in my machine to be able to do anything, not just what I want them restricted to".
Hooray, freedom!
I mean that the presence or absence of an IOMMU doesn't impact whether owners of hardware have control over their hardware.
It just means that the owner of the machine is able to limit what memory the devices in their system are able to access, in the same way that MMUs limited what memory every process on your system could access.
This is enforced by a greatly enriched TPM (and it's willingness to unwrap credentials). We have trust several layers of firmware and OS software, but the same mechanism allows us to ensure that known-bad versions of those aren't part of the stack that booted.
If I wanted secure games (and the market would tolerate it), I'd push for enforcement of something similar in the consumer space.