Are they allowed to refuse to cancel your subscription if you don't provide sufficiently valid reasoning for whatever counter-question they throw at you, or something?

Why'd you even answer the second question/offer, let alone get into an argument about it?

"No thanks. I want to cancel my subscription, please."

If they continue to ask "but why?", I'd just start coming up with increasingly unlikely reasons.

"Doctor's orders."

"Lost a bet."

"Religious reasons."

"Sacrilegious reasons."

"I'm gluten-free so I quit computers and phones."

"I literally have no money."