Nowadays, I think the the term has been largely separated from its’ negative correlation to fellatio. Personally, I never even realized the correlation until I was very far into my adulthood, and most people my age never used the word with that meaning in mind at all.

This is kind of my point - the majority population never has any reason to think about the origin or evolution of our synonyms for bad, we just pick them up from usage - usage by older people who may have racist or xenophobic intent, or may have picked the terms up by osmosis themselves. That's how the slurs get engrained in language. But I'm willing to bet, even if you don't actively think of 'sucks' as connected to fellatio, that you've used 'sucks dick' or 'sucks balls' as an emphatic. (If your emphatic is 'sucks eggs,' then you're even older than I imagine, and please forgive my ageism ;) )