Here are the things I disagree with
A: Runeglaive fixes his mana problems a lot faster than Athenes which is something he really needs in the jungle, along with providing faster clears. I think it's a bit like Elise. You can reasonably go both, but I believe in Runeglaive for both. So really it's just an opinion.
B: Thornmail does not work on anyone not stacking armor now because of the changes to it.
C: When going against an AD heavy team GA is a MUST if you expect to effectively use your ult. Even if you did buy thornmail, you will have issues when ulting the team and getting destroyed.
In summary; Your argument was very reasonable and makes sense, however I'm guessing that we are going to have to agree to disagree.