You found a heart!
Tier 6 was never supposed to exist. The top tier was 5, and then someone gave us $200, and some other people made it to 100 in several donations, and we were so impressed that we decided to create a special, super thank-you group for all of them, and we gave them a few last extra perks over the time. But that's it.
Why there must be a great tier of supporter? Because, unless we create a one-size-fits-all group (as I already argued about), then there must be a higher tier and at least one (or several) "lower" tiers. Why 100? Because we wanted to thank the people who had donated that much when there weren't any rewards for that, and it was a nice rounded number.
Why do perks even exist? Because we figured out that members would be more inclined to donate if we gave them something in turn. If you don't want any tier rewards, just tell us and we'll put you back in "regular members", we really don't mind. It's just that we don't mind giving people a few more centimetres of profile picture space in turn for them helping us maintain our sever. What's wrong with it?
Such as myself. When I donated, my mod status gave me more perks than anything I could get by donating, but I nonetheless donated $25 just to help the forums. And, you know, any amount helps. Even if it's only $5, it's $5 less Steve has to pay for the server.
By that rationale, we could have 200 tiers, from $5 to $1000 or whatever. But this is not a bussiness, we aren't a subscription website. We offer rewards as a proof of our gratitude to those who can donate. If you can't donate more than $20 and you can only make it to tier 4, we aren't going to run to your house and call you "cheap" for not giving us $100. It's not "tier 6 or gtfo". It's give us anything you can and we'll be grateful for it.
This already exists.
Tier 6 was supposed to be a special group for those who donated a surprisingly large amount of money. Lowering that thresold would make it lose its meaning. Or we would have to create a tier 7 for that purpose, meaning we'd be back to point 1.
It's called recurrent supportership.