Again, it's not aso much about server load, but about peak time traffic. People AFKing during an off peak time period will not affect your log in whatsoever. But during the peak evening hours on the relevant servers, there is a huge spike in traffic from people trying to log in, and it is that rush that is preventing people from logging in.
Think of it as a kind of DDOS attack.
It being peak hours, those AFK people are most likely going to be active anyway and while there might be a handful that are still AFK, it won't affect server load in any significant way. The problem is not the number of people in the game, but the number of people trying to connect to it. Kicking a few AFKers will not solve that issue, the only way to solve it is to add more server capacity which is itself a whole nother set of problems, see thread linked below.
Also, while kicking a few AFKers may free up a little bit of load, you are replacing a stable variable (because it's not doing anything) with an unstable one (quests, items, anything) that ADDs to server load. It was explained that doing so instead will cause the servers to destabilise, kicking everyone off the server instead, which then creates a negative spiral because you now have an even larger number of people effectively DDOSing the log in servers.
That was basically what happened during the early access days, they overloaded the game servers which caused everyone to get kicked off and so suddenly we now have up to 200k players trying to log in at the same time, which basically forced the log-in servers to overload and crash, which eventually caused the entire system to crash.
A system running at 70~80% max capacity is more stable than a system running at 100% capacity, because you then have a buffer with which to handle sudden outbursts. It's like running a marathon, you're not going to last long sprinting from the beginning.
I recommend reading these 2 threads.
http://www.reddit.com/r/ffxiv/comments/1l7n5x/why_servers_suck_a_simplified_abridged_guide_from/
http://www.reddit.com/r/ffxiv/comments/1ld2rv/further_details_on_access_restrictions/