I don't think anyone suggested otherwise?
I think the conversation might be slightly hung up on the specific example of a quill (reed stylus, sharpened bird bone, charcoal stick, brush all encourage a flowing character), nor is the only reason to prefer cursive writing to relieve hand strain, although that's true for informal long-form and for some hands and for some alphabets. My general point here is that the medium of writing tends to dictate the common forms of writing and with the common medium now being mechanical -- computer, printer etc -- we're seeing a shift in how people form their characters, if they even form them by hand at all.
I definitely find writing a nicer experience with a pencil, a fountain pen, or a felt-tipped pen - ballpoints require too much pressure to be comfortable.
Cursive is unrelated to quills and ink. Quills and ink existed for a thousand years before cursive was invented.
Cursive exists to relive the strain of writing a lot. Printing is much harder on the hands that cursive. If cursive hurts when you write, you're doing it wrong. You're supposed to hold the implement very loosely and gently, not like you're holding on for dear life.
the 'r' and 's' look especially inscrutable
The cursive "r" is an "r" with a bit on the right linking to the next letter.
The cursive "s" is an "s" with a bit on the left linking it to the previous letter.
The forms have evolved over the centuries, but they're still there. Even the weird capital F makes sense if you write it enough times. It's an "F" made without lifting the pen.
I went to public school early in life, and so didn't learn cursive. When I joined a private school, everyone was already writing cursive, and I had to catch up. It took me maybe three weeks. I remember getting g and q was hard for me. But people are able to learn new things every day.