I guess we will just have to agree to disagree.
As hart has also mentioned, the filter catches organic detritus/particles and traps them. Since they are then trapped inside the canister, they are very much a part of the water column. If you don't clean it out then all of that material will continue to decay and contribute to poor water quality. The size of the filter is irrelevant.
Also, the eheim is rated at 900gph (I'm sure with an empty filter that has no media) so it will probably only pump around the 6-700gph range once the filter is filled with media. My point, I think you're a bit ignorant about the filters if you think that will filter a 500 gallon tank on its own. To put it into perspective, I would not rely on the eheim 2262 (or the FX6 for that matter) as a single filter for any tank over 125 gallons. If I had a 500 gallon tank, I would want to put at least four of either filter on it (well, lets be honest, I would just use a sump instead).
So, I still recommend that you just increase your canister maintenance to once a month or so, instead of trying to get a filter that will "just stay running" for six months, or longer.