Even if the crowd is like 100000 strong, hardened and willing, it is not a small threat and can do lot of damage. Statements like 'no pressure on PM' are premature and foolish. If I was PM, I would be concerned given the scale of the march in question.
Secondly, crowd in Islamabad might just be a tip of the spear. JUI-F have millions of members, and have backing of political parties such as PPP, PML(N) and ANP. These people desire concessions as in escape accountability.
I do not rule out the possibility of victory of the government but this fight might turn ugly for all as well. Every fight is different.