It is not necessarily about threatening them, I look at it as can Japan fill their production capability.
(assume Japan losses Kwa trn on UK1)
Option 1 - buy 2 ICs earn 36
On J2 they can get 8 units to Aisa (8 units out of a possible 14)
J2 - bought 1 trn, 2 arm, 6 inf
On J3 they get ~10 units to Asia (10 out of 14)
bought ~ 2 arm, 10 inf (need another trn or only get 10 units to Asia out of 14)
J4 buy 3rd trn and troops and you still only top out at 12 units out of 14.
Option 2 - Buy 3 trns
On J2 you get 8 units to Asia ( 8 units out of 8 )
bought 1 IC, 7 inf
On J3 get 10 units to Asia (11 out of 11 with inf from islands)
bought 2 arm, 9 inf (save 3) - (11 units out of 11)
Then buy you second IC on J4 and you can still max out at 11 for 11, then max out at 14 for 14.
I think I did the math right, so from my pov, as Japan you want to max out your current production levels before upgrading with ICs on the mainland.
You already start with an 8 level from Japan, jumping immediately to 14 isn’t beneficial since you can’t fill 14 for quite some time.
that is 42 IPC if you are only building inf, throw in 6 tanks and 4 inf instead and you top out at 10 units.
Why not get 1 IC and 4 trns and top out at 11 and you can now get 6 arm and 5 inf (if that is the rout you go) to Aisa and it only cost you 15 (IC) + 24 (3 other trns) = 39
vs.
30 (2 IC) + 8 (1 trn other) = 38 BUT only 10 units, to get 11 you need another trn, so
30 + 16 = 46.
So it is 39 vs. 38 or
39 vs. 46.
I’ll take the 39 in both cases for the flexability alone.
Plus I like to try and get one Japan IC closer to Moscow when I have secured Asia.