Even if we cut out that the first 2 digits represent the network (i.e. Mastercard/visa) and cut another checksum digit at the end, it leaves us with 1 trillion

Even if we cut out that the first 2 digits represent the network (i.e. Mastercard/visa) and cut another checksum digit at the end, it leaves us with 1 trillion numbers. That's about 150 for every living human on the planet. Then factor in that CC numbers have an ~2year expiry date and a 3 digit CCV number, so old numbers can easily be reused. Further the name of the holder could be added as a key to the number, further increasing reuse. We won't run out of CC numbers anytime soon.