Thank you for the answers.
An important additional point is that if I decide to commit with Asiana, I'm also planning to get a credit card with them to stack miles even faster. There are some available in Japan. This will help for mileage accrual, as flying eco might sometimes not give as much as expected, indeed.
I assume that miles obtained by CC do not count towards status?
As for the thing ST vs *A, it mostly comes down to a compromise between destinations (ST is not necessarily possible on all the routes in Asia) and trying to fly "metal" as much as possible because of my eco class bookings (mostly Asiana or ANA for *A, vs Delta for ST - I try to avoid Chinese airlines is possible, which rules out MU).
Last but not least, every program is likely to be "not great" for flyers in economy. Thus, it might be more advantageous to focus on one program with status easy to obtain, then grind the miles mostly with CC expenses.
Guess I'll be waiting for a couple of business trips to see if I can fly Asiana or *A partners consistently, then decide accordingly.