To keep the Max cap increasing, attacker needs to keep burning his coins. And once he burns out, the cap will automatically scale down. In reality, an attacker, who is willing to burn his coin can always play with bitcoin network. Recent spam experiments prove that beyond doubt. On the other hand, BIP 101 only monotnously increase the max cap, without accounting for the market demand. It does not adjust the max cap with decreasing demand for space in blocks.
The exact same cost is incurred if ANY spammer wants to fill blocks. Anyone willing to accept that as a safety net should have zero problem with the exponential growth in BIP 101, since that increase in maximum block size will have zero effect on the actual block size unless they're filled. If they're being filled by commerce, great! If they're being filled by spammers, then at least there is a hard cap with BIP 101. BIP 1xx does not have the safety of that hard cap.
BIP 1xx as written also has the reverse problem. A few miners can collude to restrict transaction volume in their blocks, causing a repeated halving of block size CAP. A rational minority miner who would otherwise be willing to include all those transactions will find his block size restricting repeatedly.
I realize that these two scenarios are mutually exclusive - they can't both happen simultaneously. Further, I would agree that neither of them is particularly likely. The problem I have is that I don't see a rational reason to accept these possibilities. BIP-101 does not allow for either of these (unlikely, but terrible) outcomes. It follows a predictive curve, just like block rewards.