Re: Serious question about MANPADs
A MANPAD only has so much fuel. You cannot extend it by making the rocket larger otherwise it will no longer be portable. If done as a DIY job, won't be particularly aerodynamic or balanced either assuming it did not blow up on you! If it is possible to make it go higher, the manufacturer would have already done so.
You can in theory extend the height ceiling by sitting on top of a mountain to fire your MANPAD. You would probably be bloody cold as would the manpad electronics. However, factors against you are the reduced oxygen content at height giving the MANPAD motor less grunt. Also your window for locking on and getting the missile into the right place to catch the plan would be limited. Being on top of a mountain is no good if the plane you want to hit, in addition to its height, is a couple of miles to the side of you as opposed to directly above. This is why near airports is a high risk area, the plane is low and slow and everyone knows where the plane will be ie. lined up on the runway.
The airlines have already assessed where the high risk areas are and avoid them (Ukraine, Syria and now add Egypt to the list). They can't avoid everything in existence so just hope that a terrorist doesn't get hold of a fighter jet and simply shoot one (or more) down. Fortunatey religious fundamentalism and high technical intelligence and skillsets don't often go together!