This discussion is pretty pointless...
OK, in most books, the cover html file (which is explicitly set as this kind of file in
guide section) just contains a link (in either img or svg>image flavour) to a bitmap which is also set as cover image (in
metadata section).
But you can freely do whatever other combinations you want.
So, therefore, they cannot be directly linked. Period. Now, discuss whatever you want but it's absurd IMO.
Maybe your cover.html just contains text, or maybe it contains only SVG vector shapes, or maybe it contains three bitmaps. Or maybe you just want to set one of them (or none of them).
If you are really interested, what you could propose would be an enhancement to Calibre where, whenever you set an html file as cover type,
IF AND ONLY IF it only contained a single link to an image, Calibre automatically suggested you to mark that image as cover... As you can imagine I cannot foretell Kovid's answer, but I'll bet to "sure, patches are welcomed"

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