They're worried about what would happen if that funding were to disappear at some point (Google could choose not to renew the contract at any time and then Mozi

They're worried about what would happen if that funding were to disappear at some point (Google could choose not to renew the contract at any time and then Mozilla would have a huge problem). Part of leadership in a big organization is looking ahead and trying to mitigate potentially-catastrophic risks before they become actual problems. Mozilla doesn't want the future of their entire organization to rest in the hands of Google, whose decisions they don't have control over (and who is also their main competitor).