Waymo isn't a bet. At least, isn't a bet that is projected to be even remotely cash positive, much less having a significant percentage in Alphabet's revenue, in any foreseeable future.
All these "north star goals" and "company bets" are often pipe dreams.
Waymo has been around for 13 years now. It's bet is to have full level 4 autonomy, at least. And that is another 20 years away in an optmistic scenario.
Well, unless you only drive on sunny days in a geofenced region in Arizona and call crashing into a bus "a misunderstanding and a learning experience".
But that 20% isn't advertising is much better for Alphabet than compared to 10 years ago. Waymo is the next big bet.