Is the APS-C DOF penalty all that important?

Yes - if you are shooting at f/2.0 with your hypothetical lenses, then you cannot open up the 67mm lens any more to match the new DOF

As you point out, there is no 67mm lens f/2.0 lens. And if there were, would it have the same design tradeoffs that make people value the 100mm f/2 lens? The real problem is 50 years of lens design and user experience for a 24x36mm imaging area. There are "sweet spots" where lenses have been optimized or just recognized as having certain desirable characters in terms of sharpness, blur, and subject distance. If you like the look from an 85mm f/1.2 on a head and shoulders portrait, what lens are you going to use to replace it on APS-C? How about the look from a Nikon 135mm f/2 DC or Sony/Minolta 135mm STM? (While you can use these lenses on the smaller sensors, you will change the subject distance to keep the framing which will change the DOF and background blur. It may also mean you have to shout to speak to your subject or extend the back wall of your studio or provide more flash power.)

Eventually, there may be enough lenses and experience with APS-C to identify new sweet-spot lenses that match close enough to the old standards. But right now the easiest way to draw upon all of those man-centuries of experience is to use a 24x36mm sensor.

To most people, no. But them again 90% or more of the pixel peeping arguments on this site are not significant to most people. If what you have satisfies you, why do you care about what other people say or want?

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Erik