Hey, I have a campaign concept here, inspired by a Fear Factory song 
Let's suppose an Athar wizard has a real problem with a single-sphere deity (the power in question is only really prevelant on one Prime Material world).
Now, let's suppose the wizard in question can create a sustainable portal or conduit from the Inner Planes... let's say, the Elemental Plane of Fire. This means that the elemental plane isn't a passvie portal, i.e. just doesn't sit there, but actively pours out the element in question out of the conduit.
What would happen if the wizard could create a portal (or conduit) from the elemental plane of fire onto the Prime Material Plane? Since the Elemental Plane of Fire is by nature infinite, and the portal in question can expel the elemental property outside of itself on the destination (the prime world), wouldn't this mean that the Plane of Fire would be pumping pure elemental fire into the Prime Material Plane?
That means the world in question would soon be incinerated as the atmosphere is eaten by the non-stop flame pouring into the world... The oceans would boil away and all life on the world would be atomized, leaving only a barren, charred world. Thus, effectively killing all of the worshippers of said deity in one fell swoop, starving the deity in question of power and belief, and dooming the power to a slow death.
However.
Since the Plane of Fire is by definition infinite, and the Prime Material Plane is by definition infinite, does this mean the fire would spread across the Prime, destroying worlds across the plane? The Big Fire. Utterly obliterating countless Prime Material Worlds. The End, plain and simple.
The last thing anyone would see was the sky becoming bright... too bright, before the atmosphere was devoured... more fuel for the fire.
Which would also mean that every single Prime Material World and every Power in the Outer Planes would be in danger because some barmy Athar found out a way to destroy a Power. Imagine the devastation that it would cause, especially if you subscribe the the "phlogiston and crystal sphere" theory proposed by Spelljammer... Phlogiston is flammable, you know...
How would run with this concept? Would you try to have the PC's stop it before it stops? Or would you have the PC's doing damage control before it reaches their Power's world or even their own?
Tell me what you think.
No.
It could if you wanted it to for your campaign, of course, but it's not a necessary consequence. For one thing, even if there's an infinite amount of fire on the Plane of Fire, there's still a finite amount of it coming out of the conduit over any given finite period of time. Therefore, the fire from the conduit won't fill an infinite space until an infinite amount of time has passed. But even if the fire is coming out of the conduit infinitely fast...it still won't necessarily fill the Prime. You can take an infinite amount of space out of an infinite amount of space and still have an infinite amount of space left over. (There are an infinite number of prime numbers. There are an infinite number of integers. But take the prime numbers out of the integers, and you still have an infinite number of integers left. Infinity minus infinity doesn't necessarily equal zero.)
There are other reasons why such a conduit wouldn't necessarily have the consequences you describe. Would even pure elemental fire keep burning indefinitely once removed from the Elemental Plane of Fire? Would it necessarily spread out, instead of just being more and more concentrated in one place? Would it be able to cross space, where there's nothing to burn? Would it be able to penetrate the crystal sphere? Could the gods step in and create a break or otherwise stop its spread?
But, again, it depends on what you want for your campaign. A conduit permanently spewing elemental fire doesn't necessarily mean the entire Prime Material Plane is in danger...but if you want it to mean that for your campaign, sure.