Depending on where you are going (Europe vs Asia) there might be a range of "Y" fares on NW, so it's best to think of Full Y, other Y and B. To Asia, there NW has YLE and YHE fares that are much. much cheaper than full Ys but upgrade at the reduced full Y mileage rate (they have a $100 change fee though). To asia, I often I see NW YLEs for only about $300 more than a NW B fare (which costs 2x miles to upgrade). I'd rather spend $300 to cut the miles in half as well as to insure easier upgrade availability. As you note, Ys upgrade to Z and Bs upgrade to I. If you peruse a fare bucket tool, you will see that I is very scarce, a few in advance and then others dumped as they realize the flight wont be full up front. All of the Y's are fairly easy to upgrade if you have +/- a day flexibility or are willing to consider slightly different routings (e.g., PDX-SIN is much more definite for upgrade than SEA-SIN).
I have more limited experiences with Europe, Y fares were pricey but fairly easy to upgrade, B fares could be really cheap, often sub $1k in the fall/winter, but ofter took tricks to get upgrades. NW uses married segments, so SEA-AMS might not have I space showing, but SEA-AMS-LHR does (even though SEA-AMS is the same plane!), so you may want to consider different destinations and using a LCC or train to patch your trip together.