You are correct, we all could do better at image interpretation.
Personally though, I think it can be done without doing a full residency in both. I am sure there are good reasons that radiation oncology is no longer a subset of diagnostic radiology. I have been to the RSNA meeting before, and I was really impressed. They make the ASTRO meeting look like a traveling carnival compared to Disneyland. The ASCO meeting is similarly impressive in scale, but totally boring in the exhibit hall - no machines on display!
Not that it couldn't be done, but it is nearly impossible to keep adequately up to date in 2 or 3 specialties, especially if you try to be a generalist in each. At the end of the day, people will be drawn more toward one and end up developing that skillset more, which I think is why the fields split. The breadth and depth of both have increased a lot since. There are imaging fellowships now for just breast, for instance.
I know a guy who did 2 years diagnostic and 1 year therapy in his radiology residency. That was it - 2 fields in 3 years. He was a very good rad onc by the way. That was in the 70s though, when MRI, PETCT, and CT simulation were not widely available, if at all. There was no SBRT, IMRT, 3D, CBCT, HDR, or most of the other things that we now take for granted. Cobalt-60 was commonplace. Both fields have gotten a lot more sophisticated and nuanced since the 1970s - it's been 50 years now.
Politically, if the fields were merged, we can all guess who would get listened to more and who steers the ship. If ASTRO doesn't represent your interests now, how well would the RSNA do?