Two aspects to this.
A - How far would Luffy have made it without Zoro
B - How many Strawhats would have died without Zoro and thus making Luffy die earlier
Can't remember if Nami was saved against Buggy, probably was.
Usopp would have died to Jango and the cat brothers which would mean Luffy would at the very least have died to Lucci or Sugar.