Auto rebuild for players who are not inactive, is a better way to deal with missle spams when you pair it with a reduction in cost for APM of 50%. But not at the same rate as IPM production.
Fastest build time for IPM is one per second, barring use of items which would be a huge waste anyway.
Have APM being able to be built automatically (resources permitting) up until the storage limit at 1 per 2 seconds.
Ex:
Attacker and Defender have the same lvl 10 silos. So Attacker can launch 50 IPMs and needs to overcome 100 abms to start hitting defences.
Assuming defender has enough resources to sustain build rate, and for simplicity that attacker can instant start the next round of ipm builds. In reality getting back to defence tab will slow this down and give defender more time to build.
W1: 50 IPM vs 100 ABM=== 0 IPM and 50 ABM
50 seconds later
W2: 50 IPM vs 75 ABM=== 0 IPM and 25 ABM
50 seconds later
W3: 50 IPM vs 50 ABM=== 0 IPM and 0 ABM
50 seconds later
W4: 50 IPM vs 25 ABM=== 0 IPM and defences start taking hits
and we now reach steady state, of the 50 IPM launched only 25 get through to hit targets. So in current system it took 2 waves and then you were landing 50 missles per wave. Now there is an additional wave and the attacker is only landing 25 per wave, making the harrassment take twice as long and is twice as expensive in IPM costs.
So this makes missle spam more expensive. When you pair it with a cheaper APM, the harassment value isn't really there unless you have several planets involved in the bombardment.