Your seeming frustration about the future of radiation oncology is likely related to one simple fact: it is impossible to predict the future. One can make guesses, but no one can be sure. As far as the future of radiation therapy in cancer treatment is concerned you can imagine two extremes. #1: Somebody develops a non-radiation-based cure for all cancers and obviates the need for radiation therapy. #2: Advances are made in radiation oncology that lead to evidence-based treatment algorithms involving radiation in the treatment plan of every cancer patient. Obviously, both of these extremes are ridiculous. The fact that #1 may seem marginally more plausible than #2 does not mean that it will come to pass. The reality is going to fall somewhere in the middle.
Currently, there are a number of areas/tumors for which radiation therapy is playing a less important role (e.g. pediatric & Hodgkins) and for which it is playing a more important role (e.g. GI & organ-preservation) than in the past. However, even for areas where radiation is currently less favored, there are those that argue that this is just another example of the ebb-and-flow of cancer treatment and that with future advances such as more precise delivery (e.g. IMRT, stereotactic, etc.) or better radiosensitizers, this will be reversed. One thing that is certain is that we are facing an aging population in this country and that the absolute number of cancer patients will increase over the next few decades. The bottom line is that the exact future of radiation oncology (as that of pretty much everything else) is uncertain but it is extremely likely that radiation will play an important role in cancer treatment for a long time to come (if not forever). Said another way, multi-modality therapy is here to stay and radiation oncologists will continue to play a critical role in the treatment of cancer patients.
As far as research areas with the potential for major advances in radiation oncology (e.g. IMRT, organ motion, proton radiotherapy, radiosensitizers, systemic radiotherapy, functional imaging, oligo-metastases, stereotactic XRT, gene therapy, etc., etc., etc.), the list is long and the answers as to which ones will pan out are unclear. What is clear is that we are far from having optimized the use of radiotherapy to treat cancer patients.
If you are unable or unwilling to deal with the uncertainty of working in a fast-moving field which requires you to stay abreast of the latest research in order to best serve your patients, then radiation oncology is not for you. However, if you are excited by these attributes and maybe even interested in helping to refine or extend the uses of radiation in cancer treatment, then you should strongly consider it for a career.
[By the way, this topic has come up a number of times before on this forum and there are many informative posts about it. Seek and you shall find.]