There will be lots of other nodes next to the master nodes. Even with 10K you can do something Chris said.
The 250k amount is chosen because the amount of master nodes must be limited to get it to work properly.
Chris is clever CEO, he can satisfy all kinds levels of investors, let them make money in masternode mode in TumbleBit wallet. People with 10k coins can also make money, so do 250k whales. Good design.