The adaptive blocksize in itself is a security weakness without substantial fees. A spammer can easily make the system "adapt" to his ever increasing demands to spam it for near-zero cost and destroy it. Plus network propagation and block validation has a certain time required, you cannot go to infinity just because someone is spamming it.

So, when the time came for such a spam scenario, "central intervention" by the devs happened and they raised the fees quite higher to make every tx much more expensive.

So the solution for Monero, wasn't the adaptive blocksize (it was broken unto itself, if you expected the "market" or the "miners" to determine the fees) => but the centrally imposed fees.

Likewise Bitcoin could have 4mb blocks tomorrow morning, if, say, every tx was bumped to something like 10 cents minimum. This, by itself, would probably have us at <500kb block use right away by eliminating dust & spam, with an 8x expansion space up to 4mb.