Ok,
This wont work as you describe, lets take your concept.
What happens when your airliner arsenal plane meets opposing fighter aircraft?
It will end up very dead as it doesn't have the speed to cut and run away, it doesn't have the manoeuvrability to deal with any enemy fighters that gets within visual range, oh and its a huge RCS , therefore its going to be seen at the longest ranges by enemy sensors. And the counter point that the arsenal aircraft will engage every target before it gets close doesn't work, its never worked in the history of air warfare, look at the YB-17 Gunships from WW2 for example.
The second reason that this is a bad idea, if you buy one arsenal bird to replace for example 5 medium ranged SAM batteries lets say Patriot/SA-10 type systems, the enemy has 1 target to strike not 5, 5 Sam batteries can cover a larger volume of airspace than a single arsenal aircraft, sure the arsenal aircraft can move quicker than SAMs but the volume of airspace covered is still constantly a 5th of the airspace 5 SAM batteries cover.
This is further exacerbated by the fact that the enemy only needs to counter 1 technology airborne interceptors, where as an integrated AD system comprising of interceptors and SAMs in layers complicates the enemies offensive actions by making sure that the enemy has to have counter air and SEAD assets available as part of a strike package.
There's also the issue of putting all your eggs in one basket in two senses, if you have arsenal aircraft and there's a technological break through next year that nullifies that aircraft and leaves you defenceless, and secondly in a tactical sense, if you put a huge number of AMRAAMS or Phoenix onto one aircraft that's no other fighting ability, if a stealthy 5th Gen type knocks it down then you have lost a huge amount of your missiles in one fell swoop.
This idea works great in fiction and looks attractive on paper but in the real world it doesn't work am afraid. Even the USAF is buying the F15EX with the capibilty of carrying I think 16 AMRAAMS and using it as a missile magazine to provide magazine depth to 5th Gen fighters but it can still fight all but the most capable opponents.
Regards
Butch