Never charge for bandwidth
It seems like you've been talking about charging for bandwidth for a long time but haven't actually done so. Some people (like me) have built high-bandwidth applications on thin margins that could be decimated by bandwidth costs. That's probably my fault, of course, but you not charging for bandwidth is a massive advantage you have over every other competitor in the field. Today, I couldn't imagine hosting my business anywhere else. I absolutely love DO and am planning many more droplets in the coming months. Please don't change a thing!!
Could offer an option to throttle bandwidth rather than charge for it too, like a cell phone plan. I think that would be more reasonable since it punishes those using a way-too-large portion bandwidth without giving a perverse incentive (DO is not paid for punishing you).
What kind of application do you run to reach such high amount of traffic on a single droplet?
Out of curiosity, what do you do to even get close to such a high limit?
If you're serving cacheable files via web, you could proxy your website through Cloudflare (seems to be unlimited bandwidth even on free plan unless you're under heavy DDOSes) — that will lower direct bandwidth. For example, one of my static files host working via Cloudflare used directly from DigitalOcean VPS only 250 Gb last month while users got 5 Tb downloaded.
Unmetered bandwidth @ DigitalOcean was good feature though.
My GUESS is they use those limits when needed. Example: If you have 3TB of traffic on your droplet but you go 3.3TB one month but overall you are around their limit it isnt big for them. The issue is, they can use those limits when someone has 3TB but consistantly uses 12+ TB. I am with you, I dont want them to change it but I am also guessing this is just protection of their services not a hard limit.