Seems easy to lose the plot on this. Again, the PM is a Program Minimum, after all.
I don't think many people will argue that the swing is an excellent drill for conditioning - so that's probably a non-issue.
So, the issues that come up seem to imply that the TGU is lacking. If you were designing a minimalist program and trying to choose only one kettlebell exercise other than the TGU, what would it be?
Only one because it's a minimalist program - in this case one strength drill and one conditioning drill.
Kettlebell only because after all, it's in the book Enter the Kettlebell.
Sure, you could string together a circuit with a clean & press, windmill, and front or overhead squat, but is that still a PM? You can add more exercises to "fill in the gaps", but again, is that still a minimalist program?
Keep in mind too, that ETK is aimed at beginners. Yes, it goes beyond that stage, but with the detailed instruction, it's clearly aimed at people with little kettlebell experience. So, your one strength drill can't be too complicated.
If you're trying to hit as many body parts as possible, while keeping them working as one unit, what would you use instead? Remember, only one, and kettlebell only. I'm not that experienced, so there may be a good one out there, but I can't think of one.
I'm not really challenging and saying the TGU is the only or even the best option. But, it's certainly a good one and covers a lot of bases for a single exercise, and many people are reporting great results. Personally, I went on the ROP for a month or so and I could tell the volume was too much for me - too much for my sketchy shoulder, and took too much time. So, I'm back on the PM. I'm slightly tweaking it, but just scheduling really. I'm not really adding any more exercises (I often finish with a loaded carry, and sometimes ab-wheel rollouts or planks).