There are lots more choices out of DEN than the carriers you mention. AA, DL, AF, BA, LH, and TK all fly out of DEN too. I can choose many airlines to fly to many places. I believe in ATL there is the same set of airlines (if not more).

You seem fixated on nonstops. If that is the case, that is fine. It is what you value and no one should give a hard time about what you choose to value. Personally sometimes I care, sometimes I don't. Non stops are very nice when I can get them. But I do not always use that as my primary decision making point. Particularly if i have to change planes no matter what. When I was flying heavy I usually had to change planes. For instance I would often fly DEN-DEL. I had a choice - change twice or three times. If I flew DL it was three. If I flew AF/UA/LH it was two. I was ambivalent since the itineraries were within a few hours of travel time. I often chose class of service over plane changes. The least important thing when I booked was alliance/airline. I did prefer to stay *A/ST but when it came down to it I would fly BA if it made sense.

If I flew domestic heavy I would probably have a slightly different opinion. Even then, gold on both UA and DL would be better than Plat on ether airline alone. For me having choices was more important. Sometimes my time was critical and I needed to be there immediately. Sometimes I could take a second leg. UA still has a segment requirement for status. That meant I was often better switching planes on UA. On DL it didn't matter.

I assure you multiple airlines can take you to your destinations out of ATL. I have no issue if someone tells me the fly DL because they prefer it above others. I understand the allure of non-stops. Many times I prefer them. However, I won't agree that there are no other choices. There are always choices. No one is a victim here. As a potential hub captive out of DEN I found that with little difficulty I could get around. Even domestically.

Going all the way back to the original question - diamond is indeed useless to anyone who has the willingness to purchase the class of service and schedule one likes. In your case you could fly DL to the cities you mention but another carrier for international flights. I often did something like that. The last few years it just so happened that I landed status due to the sheer volume of travel. I spent more time on partner airlines than I did on DL or UA metal. I found it to be both cheaper and easier to jump AF to CDG and then another AF flight to wherever I need to go. Sometimes DL would have the same itinerary, but it was almost universally more. same goes for TK and LH. Both had great networks and better pricing.

I was worried when I did this that I would miss c+, early boarding, and the other perceived perks of status. Personally I did not. I found that flying DL or UA to LAX, SEA, ORD, or JFK would allow me to jump a flight on an international carrier that had better service and a better hard product. It was cheaper and easier to book two tickets in those instances than to try and book through on DL or UA. The positioning flight was nothing and I did not miss a thing.