Some numbers:

Assume it takes on average 30 seconds to verify 1 MB of typical transactional data (k =0.5 min / MB).  Since T = 10 min, this means the maximum average blocksize (network capacity) is limited to:

    S_effective   =   T / (4 k)   =   (10 min) / (4 x 0.5 min / MB)
                   = 5 MB.

QED. We've shown that there exists a limit on the maximum value of the average blocksize, due to the time it takes to verify a block, irrespective of any protocol enforced limits.