The funny thing is that this system with bots, you mentioned, was sustainable from the beginning of Steemit and worked quite well when there were only for few whales and orcas. The only thing that has changed is a long waited "quality" content creators and user support. Somehow that was not very well welcomed by old-school bot users.
Probably this is what is not sustainable - draining reward pool with bots or massive user support for a quality content.
Is it possible for a few bad actors to drain the entire rewards pool via bots?
If so, how could the witnesses resolve this issue?
Yes. But they are not going to do it, because then it is too visible even for a newbie users.
When you limit something for a legit user then with bots it's useless. So, I do not have a solution yet.
They need to do something, because if what you say is true, one person and their bots could possibly take up the majority of the rewards pool.
I guess there's more coming this year. Stay tuned.
The potential reward pool abuse is a whole other issue and it is also an important one. My personal take is that we need to get better at identifying and self-regulating abuse. It will need more users to take a more holistic and benevolent view toward building the community and the platform.