In the case of an 'invalid' transaction whereby the state transition of the execution is deemed to conflict in some way and reverts, the transaction itself is actually valid; gas is consumed, tx included in the block with the affected gas payment deduction.
The intrinsic gas of 21000 exists here as the baseline cost for a transaction (the cost for verifying the ECDSA signature over the transaction) and is the minimum amount charged in any transaction. If the transaction is not valid due to an incorrect signature (malformed or else), the miner rejects the transaction completely, it does not enter the tx pool and is not propagated through the network.
However in this case of an invalid transaction signature, the miner has to attempt to verify the transaction and regardless of failure or success, the computation is spent. Could someone not flood the network with malformed transactions, the miner none the wiser, and force wastage of computational resources on verifying fake signatures?
If that is the case is the verification step of a miner to deem a transaction valid/invalid a sunken cost?
If this is expected, why even charge the intrinsic gas other than to recoup previously wasted (network-wide) computational resources at an arbitrary rate of 21000 gas per successful tx?
Or have I misunderstood the computation that the miners perform prior to adding transactions to the tx pool?