Since the bubble expands faster than the speed of light, it would not be visible in the CMB. The reason that there should be billions of years left is just a statement about probability. Given that the universe has already lasted for billions of years, it would be very unlikely for it to end tomorrow (in fact the odds would be about one in a trillion). So at the very least the universe should last for at least about as long as it's lasted so far (though it could last much longer).