Your reasoning isn't flawed but one big reason why I still write some checks is this: they're accepted as payment for free by the receiver where most electronic methods are charged. For example, my apartment's management company charges me $12.95 for a one-off direct debit payment, $6.95/payment for an automatic recurring debit payment, $32.50 for a payment made by debit card, or a whopping $109.55 for a payment made by credit card.
(This means that the free--or at least "included in my rent"--payment option is for me to write a physical check and put it in the night drop box the day before rent is due and then their employees have to deal with the check, as opposed to the computers handling it all. Since the night drop is right there on my way out the door, the convenience factor is still high for me.)
I don't like checks but I flatly refuse to pay money to pay a bill. If I owe you money, you will figure out how to accept it in a way that doesn't cost me more. What I owe is already figured into the balance; charging me more to hand you money is obnoxious. My apartment company isn't the only company like this so I still hand out about 20 checks per year. Fortunately, my credit union gives me one box of checks for free every year and I only use less than one book of them per year so there's no added cost there, too.