If there is reasonably common non-DRMed software that can play it today (e.g. VLC), running on a operating system common today, you probably won't have too many issues just emulating said OS and using said software.
DOS 5.0 was released in 1991, over 25 years ago. There is a rather astonishing number of JavaScript based emulators that will let you run it in a browser. Even if the next 5 years of browser development break all of those emulators, getting a 5 year old portable version of Firefox that will run on current hardware isn't difficult.
I don't think decoding video that can be played with VLC today will be a problem in 30 years, and I also think that 30 years after that, all you might need to do is another level of nested virtualization. If we assume that there will be sufficiently well working emulators capable of emulating a 2017 machine in 2047, just like there are such emulators for 1987 machines in 2017, and that at least the most common binaries will somehow be preserved, you should be able to recover most reasonably popular video formats in perpetuity.
Compare that to analog material that rots and requires incredibly delicate handling by experts, while with digital media, you can let anyone try since they can't break anything. And finding a copy of a popular OS and player binary might prove easier than finding a projector/VCR that can handle the ancient format.
Even if you were to recompress every 30 years, the loss will likely be less than from physical degradation of physical media. However, if you have a lot of video in one canonical format (down to using the same encoder and settings, so you don't have to worry about weird edge cases), you can just preserve a copy of the player and every 30 years, you make sure you can either port it to a current OS, or virtualize it efficiently.
DOS 5.0 was released in 1991, over 25 years ago. There is a rather astonishing number of JavaScript based emulators that will let you run it in a browser. Even if the next 5 years of browser development break all of those emulators, getting a 5 year old portable version of Firefox that will run on current hardware isn't difficult.
I don't think decoding video that can be played with VLC today will be a problem in 30 years, and I also think that 30 years after that, all you might need to do is another level of nested virtualization. If we assume that there will be sufficiently well working emulators capable of emulating a 2017 machine in 2047, just like there are such emulators for 1987 machines in 2017, and that at least the most common binaries will somehow be preserved, you should be able to recover most reasonably popular video formats in perpetuity.
Compare that to analog material that rots and requires incredibly delicate handling by experts, while with digital media, you can let anyone try since they can't break anything. And finding a copy of a popular OS and player binary might prove easier than finding a projector/VCR that can handle the ancient format.
Even if you were to recompress every 30 years, the loss will likely be less than from physical degradation of physical media. However, if you have a lot of video in one canonical format (down to using the same encoder and settings, so you don't have to worry about weird edge cases), you can just preserve a copy of the player and every 30 years, you make sure you can either port it to a current OS, or virtualize it efficiently.