If bilinear filter is so great and makes old games look so much better, then why is there not a standalone device similar to the SLG 3000 for scanlines that just adds bilinear filter to the image? This would be technically very easy to do, so why hasn’t it been done?
Simply put, because there is no demand for it.
The market for such a device is videophile retro gamers, who overwhelmingly despise bilinear filter. That’s why such a device does not exist and has not been proposed. The target market does not want it!
The people who spend the most time and effort on this hobby don’t want such a device because bilinear filter degrades the image.