I already told you, he has like 3x the amount of FF that fight, and he would NOT have used that many if the zerg backed up.
To really think you can wear that force of sentries energy down before your expansion is in the fire is starting to get amusing.
In ur example you say that you wait untill FF wears off, then he will be afk and not make any more? very good, smart. I didnt think of that!
Lets say you back up, the protoss will obviously start going down towards the natural for attacking / placing FF using the hive as a rock.
I know i would. Any attacking player would advance if you "back off a bit". Placing forcefields further in the direction your army is located while moving.
To think you will be able to stop him from doing that if you run away out of his fire range is moronic.
There is nothing stopping the protoss from making a forcefield wall further ahead in advance to be able to take out the expansion if you flee out of fire range.