I mean this sincerely and not as snark: if this is a question you have to ask, just use Nacl; don't design with ciphers yourself. Since there is a "right answer" to this question and a "wrong one", "convincing" doesn't seem like a good use of anyone's time.
The right way to learn about cryptography is to start by learning how to break it. If that's something you're willing to sink time into, try this thing we set up:
Hey Thomas, does anyone at Matasano still review submissions if someone wants to submit them for a particular programming language? I'm looking to establish myself as the luminary crypto nerd of the furry fandom :3
I don't know why you got downvoted. Maybe it's the furry thing. Cryptopals is still ongoing (there's a set 8 in the works, all elliptic curve attacks). As for posting the solutions: we're doing that, too, in the abstract, but we're all busy and every time we bring it up a bunch of people say "noooo don't post solutions".
We are still working on new sets, though obviously the rate of new sets is pretty low. The mailing list is basically unmonitored at this point, but everything we've got is on the site. (This is a vast improvement on the previous state, where we regularly failed to set out challenges to people who emailed us, due to overload.)
"Because that is what people who know more about this than all of us combined have come to that conclusion"? Sounds snark but I mean that's what it boils down to anyway.
You can cite meta-analysis, like in medicine. "The people who studied this concluded that using this method has a higher chance of bad side effects and lethal complications than using the suggested method".
The right way to learn about cryptography is to start by learning how to break it. If that's something you're willing to sink time into, try this thing we set up:
http://cryptopals.com
It's totally free and by the send of set 3, you'll have an appreciation for block sizes.