An attacker like the PBOC itself has trillions of dollars to dump and bloat the network and keep it at the maximum, so even if you set a limit, you would end up with a blocksize that's too big for most people and then the nodes start dropping like soliders in the d-day.
If these "money making machines" want to destroy Bitcoin, they can do so now. An 51% attack costs about 600-700 million dollars at this moment. The PBOC would have no problem with that. So why go the hard way and spam the Bitcoin network during 10 years to achieve ~3,5 MB base size if you can destroy it instantly?
As the blocksize growth - as already said - is linear, I think the importance of this attack vector is negligible.
@DooMAD: I have, however, a slight update proposal:
Change
(TotalTxFeeInLastDifficulty > TotalTxFeeInLastButOneDifficulty)
to
(TotalTxFeeInLastDifficulty > average(TotalTxFee in last X difficulty periods))
with X = 4 or more, I would propose X = 8.
The reason is that totaltxfee can have fluctuations. So a malicious person/group that wanted to increase the block size could produce a "full block spam attack" in a difficulty period just after a period with relatively low TotalTxFee.
Regarding an upper blocksize cap (@Carlton Banks): In the previous proposals we discussed it made sense for me because the proposed block size growth was exponential (+5% is the last one I remember) and that could have lead to an uncontrollable situation in the future. I think this proposal is conservative enough that such an upper cap isn't important. I also wouldn't totally oppose it.