Sorry if I totally misread but were you actually referring to current year? Otherwise, it wouldn't seem to make much sense because the team who wins the championship in current season already gets credits.
Calendar year, as in the entire calendar year--not just one team. (or perhaps the year could be adjusted to coincide with the release of the current 201x season)
Each calendar year, there would be a "race" for the highest manager rating in the following eight categories:
ATG
201x
90s
80s
70s
60s
86
Total (of the above)
Any team started in the calendar year would count. It could stimulate interest in some of the lesser played sets since it would be easier to compete in them. It would also stimulate participation in the (normally slow) winter as the races come to a close. It would give everyone a chance to compete on a level playing field against some of the old-time veterans since the races start fresh each year. (Total points are, of course, still maintained.)
SOM sells more product; leagues fill faster; more excitement and reward for those who play a lot. Everybody wins.
And if it's really too hard to import all the old seasons, then start fresh.
This begs the question, why wouldn't SOM want to do this?