I've asked about this several years ago, then forgot about it, and now in 2019 it really seems like this has become essential for Roku to implement. Screen mirroring/casting has become a commonplace feature, but Roku's compatibility is terrible, and the common response to "buy a Chromecast" is one that should set off alarm bells at Roku HQ.
Roku relies on Miracast for screen mirroring, but Miracast is poorly/unevenly supported in Windows and Android, and not supported at all in iOS or MacOS (which rely on AirPlay) or ChromeOS (Chromecast only) and in Linux only by wizards. Since there is no universally available mirroring technology, it makes sense to at least fully support the universally available casting technology, namely DLNA. Fortunately, through @RokuChris's Media Player app, Roku already has DLNA support; it just needs to be extended to advertise itself as a renderer. Since this is a standard part of the DLNA specification, I have to assume that it's quite feasible. Now, this would require that a use start up the Roku Media Player app before casting to it, but that's better than nothing. Much better would be for this to run in the background, much as the screen mirroring Miracast function does, popping up an authorization request whenever a device attempts to cast to the Roku ... this, I imagine, could be a much more involved coding project, so I imagine that it makes sense to start by simply having the RMP app advertise itself as a renderer.
Would only a few people use this? I don't know, but I'm comfortable asserting that it would be used by more people than other features that Roku has added, like Guest Mode. Heck, Google has sold tens of millions of Chromecast devices, so clearly there's user interest in casting. There's also no license fees for DLNA, so the only cost is coding.