Resolution isn't the only part to consider, the amount of detail you can store in that resolution can differ widely. With the push towards enhanced colour spaces, higher framerates, etc. there's plenty of need for better compression technology.

Besides, for a company like Netflix or Youtube, shaving 5% off the file size without a drop in quality can literally mean millions of dollars saved per year.

>> So, complete stagnation is viable.

If you consider compression from an information theory point of view, for any given measure of "quality" there is theoretically a limit to compression. It's not clear where the limit is, or how close we are to it, but because it does exist stagnation is inevitable eventually.

As it gets harder to make gains it will become less of a business proposition and more of a research/academic thing.