The effect does not replace the weapon's ability.
Each of these is an individual permanent. If one is destroyed, its contribution to the weapon's damage would be lost.
They do not affect any flown weapons, only the weapon you have equipped.
Lastly, while your weapon could potentially ramp up damage very fast with several of these in play, it is also quite easily disrupted.
-I was basing this off a comparison with acceleration and overdrive which give a ramp up every turn and do not reset due to failed attack, delay, or freeze.
With the exception of titan, every weapon would be susceptible to shield counters. Even just 1 miss, freeze, or delay effect will reset the damage bonus to zero on all of these.
While a single counter doesn't help keep a card from being OP, there are a very significant number of counters available
For instance, fog and dusk shields would both ensure that you would only get 1 round of bonus from these on average.
Bonewall, phase shield, dissipation shield, wings, and procrastination shields would stop them dead in their tracks.
Ice shield would also be fairly effective.
And then there is also any form of PC which could be used to destroy the weapon. And, even if the weapon is immaterial, you could still destroy these cards to remove their effect. So PC works well as a counter.
Lastly, ice bolt and any of the voodoo doll freeze / delay combos would also do the trick.
I can certainly bump the cost a little. I originally used acceleration and overdrive as a rough guide when constructing the card. I left the cost low to try and make it useful in a wide range of decks, but I want the stacking to allow more dedicated strategies to be useful as well.
What seems like a more reasonable cost?