>Even if you were willing to pay for it, many others might not be.
I'm not convinced that there is a way that google maps could be as big as it is without ad revenue. Most people just wouldn't pay and the only reason that you would consider paying now is because the infrastructure that ad money has paid for has made it worthwhile.
a) Ads on google tend to be much less annoying than from other vendors because google maintains relatively high standards for how it displays ads. I consider it a reasonable trade-off and don't block ads on Google services - they are usually relevant and not overtly annoying or distracting.
b) the fastest possible growth isn't necessarily the highest good. You don't get a better plant by trying to pull it up out of the ground, and things that are grown fast don't always last that well. Wikipedia doesn't have advertising and while it's subject to many criticisms I would argue that its slower organic growth makes it more sustainable over the long term.
b) Not my area of expertise, but google doesn't fill half its website with a plea for donations once a year. As a layman it doesn't appear to be more sustainable than google. What is your argument for the sustainability of donations over ads, ignoring the obvious differences of these two companies?
To be clear, I agree that ads are not the ONLY way, just the most effective way for many companies. I think that there is no way that google could so easily steal market share from both Garmin and TomTom without ad revenue.