> AFAIK none of that DRM makes use of my TPM

Right, because plenty of modern machines do not have a TPM or do not have it enabled. There's little sense in using a TPM for some users when many others can bypass the TPM restrictions by using hardware without.

> What kind of DRM are people worried the TPM will enable, that we aren't already subject to?

Over protective applications which refuse to run when a machine is deemed "insecure". I don't have an example of this on PC, but there are plenty of Android apps which refuse running on rooted devices.