I think that it would be the wrong way to go for Google to worry too much about these competitive threats. Bing has a long way to go to catch up, in spite of the huge push that Microsoft has been making, and I am not at all sure whether Tizen is going to gain any traction in the market.
Mind you, I am not saying that Google can afford to slack off. Their strongest move would be to keep pushing forward and leave the competition in the dust behind them. When companies get too concerned with what is coming up behind them they tend to get too cautious. My advice to those Google execs would be to get a good nights sleep as to be rested and creative in the morning.