I strongly believe that digital distribution of movies will inevitably become the mainstream choice. There's no way around it. The young of today are not interested in physical media, simple as that.
About bandwidth, I actually don't think it will be a big problem. Technical solutions will arise that ease the distribution, such as legal p2p networks, and network capacity will improve. The bigger problem is the business side of things, and perhaps the biggest problem will be copyright legislation. It's incredible how ingenious innovations can be thwarted because old geezers passing the laws have no clue of what is going on. For example, here in Finland an ISP is offering a digital set top box with 5 terabytes of storage space in their servers. You need a minimum of 8Mbit connection to view your recordings, but the upside is that you can record as many channels simultaneously as you wish, and you can set up recordings via the net or wap. And what is the problem with this? Well, the quirks of the new copyright law in Finland (passed in 2005) dictate that you can't transfer those recordings to your own storage, you can't "record" past programs (although they probably exist in the servers) etc. The ISP has to dance a pretty complicated routine around the limitations in the copyright law to be even able to offer any kind of service. Technically it would be possible to offer an experience that is vastly superior to the current situation, but legally it's not possible.
Anyway, all physical media are slowly in decline, but it will take a long time before the situation reverses from now. Even though you can get vastly more convenient services digitally, the flipside is the complexity of those services. And until the usability of those services equals or surpasses current alternatives, there won't be mainstream transition.