I agree with you.
RAW, Haste lets you take an Attack action. Literally - "That action can be used only to take the Attack (one weapon attack only), Dash, Disengage, Hide, or Use an Object action." That directly implies "That action can be used only to take the Attack (one weapon attack only) action."
So Haste - RAW - is saying you are taking an Attack action. That can seem to offer a chink through which a cantrip might enter - but then Haste imposes a very specific constraint on that action - "one weapon attack only". It does not then matter what else you might ordinarily do with your Attack action - turn the King into a pear, for instance - none of that matters! All you can do is use a weapon to make one attack.