Actually back in 2011 when Linus announced his desire to end with the 2.6 series and move to 3.0 once the kernel hits its 20th anniversary it seemed to me a good cut-off mark. Why don't they just move to 4.0 once the kernel turns 30? Like 4 would stand for the 4th decade of the linux kernel. Then the point release would stand for the year it was released from 0 to 9 and then the third point release would stand for all the sequential kernel releases in that year.
So it would look like this where 'a' would stand for a release in the 2020s, the second 'b' would stand for the fourth year of the 20s (ie. 2024) and the 'c' would signify the second stable release of the kernel in 2024. I would assume that there would be no more than 5 releases in a particular year. If a new kernel is started at the end of the previous year and is released in the next then 'b' is bumped to the next number and 'c' goes back to 1.
This versioning scheme definitely won't reach crazy numbers. We definitely will be long gone by the time the kernel will be in the twenties The negative aspect is the lack of simplicity at a glance, it has more dots. Also there is this discrepancy between the actual anniversary being in 1991 which puts the previous year in a bit of dilemma but I think that can be ignored and just consider the entire decade from 0 to 9 as to how many years the kernel exists.