Even if there are ways to get old systems to look good and play well on newer HDTVs, it's usually easier and cheaper to just use a CRT instead. Would you get every system modded, or use a CRT that people are giving away for free every week? Ordering an expensive upscaler, or use a free CRT? Having to search for specific HDTVs that are better compatible with old system, or pick up any CRT for free?

Usually people stick with consoles for authentic hardware experience, even using flash carts for convenience on real consoles so games will play authentic. If you just want to play games and don't care about authenticity, there is the option of using an emulator and just hooking up your PC/laptop to the TV.