The bulk of all Nitrobactor and Nitrosomonas are located in filters because of the constant supply of oxygen and food brought to them. If you were to remove the filter with it's bio-media capacity and rely strictly on bacteria that is on gravel and tank objects you would struggle to keep ammonia and nitrites down even in a mature tank unless you had a very small bio-load of tank stock. Even with 50% WC's. The higher the bio-load in a tank the more bio-media is needed and in a filter where it can function at it optimum capacity.
Filters are needed for much more than biological filtration. In a high bio-load tank, even one heavily planted, there would be a build up a of detris, etc., from both plants and fish that even a 50% weekly WC couldn't remove. I'm talking about planted tanks with substrates and not bare bottom tanks with plants in pots.
My 220g is 100% planted with mostly fast growing stem plants. It has a heavy bio-load of fish IMO and I have to feed generously due to the sheer amount of fish but more so because of the amount of bottom feeders and algae based eaters of various species. I use 4 Fluval 406 canisters on that tank that are cleaned every 4 weeks. When I clean them the vast amount of sponges and the poly fill I use in them are very dirty. If I don't clean them on a 4 week schedule nitrates build up in the tank regardless of my weekly 50% WC's. If I didn't have those filters removing all the detris, etc., from the tank it would all remain in the water and substrate and my water would be anything but pristine over time. Even large weekly WC's wouldn't be enough. It might work if there were very few fish in the tank and feeding was light but even then I have my doubts. Each tank is unique and varies in so many ways you can't make one general "this works for all tanks" philosophy.
I have 2- 55g tanks, one contains all tiny nano fish, the other contains 3- 8+ inch fancy goldfish. Each tank as a Fluval 406 canister but the GF tank has 2. Both tanks get 50% WC weekly yet the GF canisters need cleaning every 2 weeks and sometimes need 2 50% WC's weekly due to the copious amounts of waste and ammonia they produce. In this instance I probably could run the nano fish 55g with a large HOB but the GF 55g needs every bit of the filtration, both biological and mechanical that 2 canisters provide. So it's clear "The large, weekly water changes, even in a tank with a heavy bioload would be enough to dilute any forms of nitrogen left in the tank to the point they wouldn't harm the fish." would not be correct. You have to look at each individual tank, stock, and set up to determine what type and amount of filtration would be needed along with weekly WC's to keep water at it's best.
