Isn't that essentially what happened? In the 1930s, the channels were allocated at 6 MHz spacing, but only needed about 3 MHz of bandwidth for the luma channel. There might have been some foresight involved, but the gaps between channels also allowed for cheaper, less precise tuners. Then in the 1950s they added a 1 MHz chroma signal on a 3.58 MHz upper sideband, expanding the channel bandwidth to just under 6 MHz.