The use of copyright as an unregulated monopoly is not a necessary feature of the entertainment industry but it seems very few can even conceive of alternatives.
Therefore, competition between streaming services means that each service will have its unique mix of exclusive offerings (including original proprietary material,) non-exclusive mass packages of less popular properties offered by owners to all services, and public domain products. In order to have a complete library, the consumer will have to pay more for multiple services. It seems obvious that the streaming services are aiming at a situation similar to premium cable today. If you want both Game of Thrones and Dexter, you have to pay two premiums. This is why they're experimenting with original material.
The thing is, consumers who don't want to pay more money will not have a satisfactory choice in opting for a DVD service only. First of all, there are licensing issues for DVDs as well. At this point, this is mostly a delay in availability, especially for original premium cable offerings. The first season of Homeland is just now available for instance. Second, and more importantly, the difference in costs for streaming versus mail DVDs means that the companies will not continue to invest the same resources in the mail service.
They will not divest themselves of the DVD service most likely, but they will purchase many fewer copies of midlist properties and very few copies of specialty properties. This will result in very long waits for DVDs. If the conservatives succeed in abolishing the USPS, of course, the new privatized service will cost considerably more. In any event, consumers will pay more for less, with some material unavailable and other material on waiting lists.
The mass purchase of just a few properties is already the province of services like Redbox. In the end, even the most popular movies may be unfeasible on mail-order DVD services. At that point, multiple-disc orders for things like TV series may be the mainstay of the mail-order services. Given delays in mailing, even if the USPS survives, this will be an effective rise in prices.
PS When Turner Classic goes online, the situtation will get even tougher for the consumer.