It will "fl[y] through the windshear fine" as long as the downdraft does not exceed the climb rate, and/or you have enough time and altitude. The problem is that you don't KNOW how much time you have, or how intense the downdraft will be. That's why you press TOGA again, to get max performance from the airplane. When through it, press LNAV and VNAV to continue on the Missed Approach. FCOM description of TO/GA switch operation:
Push (on approach with flaps out of up or glideslope captured) -
• activates A/T in THR mode with GA reference thrust limit displayed.
Thrust adjusts to provide a minimum 2000 feet per minute climb
• selects TO/GA roll and pitch modes
• activates F/D with no A/P or F/D active
Second push - activates autothrottle in THR REF using GA reference thrust.