(disclaimer: summer intern here) It doesn't matter how MOCO spends money. If no one want to bid the search engine list on Firefox, then most of the revenue will be gone and MOCO cannot operate anymore. FxOS market is not going to generate a lot of revenue at the moment. If we talk about investment, it will take at least a few years to make good revenue. And that revenue will still be really low compare to what Google, Bing, Yahoo and Amazon are paying combined.
If Google thinks they have 95% they probably can give up on us, but I don't think that day will ever come in the next 10 years. Also, while MOCO is a corporation, its mission is not to make money. MoFo came up with MOCO because MoFo members wanted to be good citizens. They wanted to pay taxes to avoid stupid tax allegation. Read Mozilla history and you will have a good laugh.
If we fail, we will restructure ourselves. You can't guarantee Google will be around forever. We just have to do what we have to do today. If we are always worrying about money, we would not be building anything. There wouldn't be any startup because half of their customers will either move on or use another service.