Bungie revealed a lot to us today about how light level and attack/defense will work in TTK. Notably, damage given/taken will no longer be linked to individual items and instead will be determined by an average "light level" across all weapons and armor.
In the stream, we were shown that year 1 weapons will have a light level of 160, and this will be fixed (ONLY year 2 weapons can be infused to increase light level). However, we need to stop thinking of this number as an individual attack level and instead a small number as part of a greater picture.
Other weapons/armor were shown at level 280 and 290, and I would assume there's still hidden raid gear that will probably be 300. However, let's just go with standard level 280 vendor gear.
If you equip a level 160 fatebringer and then level 280 everything else (special/heavy weapon, helmet/gauntlet/chest/boot/ghost/class item) your light total will become 160 + (280 x 8) = 2400. Your average light level will be 2400 / 9 = 266.66 (repeating, of course). This level will determine your damage output.
Compare that to having all 280 level gear: 280 x 9 = 2,520, and 2,520 / 9 = 280.
So, your average light difference will be around 14, being level 266 instead of 280. This is a < 5% difference in damage output.
So, while you won't be absolutely maxed out, you can still be quite effective with year 1 guns! This seems like a good compromise on Bungie's part and leaves me satisfied with the current upgrade plans.