It is not DRM that enforces it. In fact, I don't think you understand what DRM is.
If I write a proprietary video player, using a proprietary codec, and I stream you videos that only work in that player, that is not DRM just because I was too lazy to provide you a local caching method.
DRM is not "make it inconvenient for me to use with something else". Digital Rights Management is verifying that playback is appropriately licensed. That's it. A key requirement of any DRM schemes is "make it inconvenient to copy" and "make it inconvenient to play with something else that might allow copying". But that doesn't automatically mean that anything difficult to work with is magically DRM protected.
Your question and argument is that DRM is substantially impacting bandwidth, which is patently untrue.With or without DRM, the volume of streaming traffic will be effectively the same. The vast majority of people do not and will not cache content unless their playback/media management system provides that functionality. Streaming is here to stay because it is convenient. Spotify, iTunes Radio, Pandora, etc are destroying the MP3 market the same way iTunes and Amazon MP3s destroyed the CD market. Because the convenience of instantly playing something unplanned and on demand is more desirable than convenience of carrying around a curated library of personal favorites.
Yes, there will always be room and a need for hybrid approaches for streaming vs. local cache, and the market is experimenting with those business models and technology patterns now. But again, the impact is not driven by DRM, but by how people expect to engage with their media.