I'll also assume good motivations on this in that they want more people buying and especially don't want to get stuck with having to split families into separate middle seats.

One of the elite perks is getting to select a seat on purchase (as long as it's not in the first 3 rows or exit row). I usually go for 5A (with row 4 you can get reclined on). Then there's the upgrading to the first 3 rows or exit row in the 24-hour check-in window, if there are any left.

Most flights I've been on recently have looked 90%+ full, though in some cases it probably took some real discounting to get to that. YMMV.