It's not a linear scale, it's just that we're still rounding to integers, so we're going from 7 to 5 instead of 4.9 and then to 3 instead of 3.5. Since we've long since departed from having node names correspond to any physical dimension or even a meaningful composite of critical dimensions, the only thing wrong with sticking to simple integer naming is that we might run out of smaller integers before we run out of ways to improve silicon fabrication.