I personally would like to see a block size increase eventually. And I imagine in time miners will signal in agreement to such a proposal, as will validating nodes.
But Hearn's post was a post mortem for bitcoin. Without the change he considered it a failure.
You are right it would be a small change code wise. Just as increasing the issuance cap, or even adding a perpetual block reward would be. But like those, it was a fundamental shift that would change the entire system at a fundamental level. I would say it was more about tearing up the social contract, game theory, economics, and security and starting over at the time. I do not think the protocol was ready for such a change. It may never be.
But I also think keeping blocks smaller and thereby protecting bitcoins decentralization is worth remaining conservative.
Yes, it seems that Hearn was wrong (so far) and premature in his actions. I'm just saying he wasn't advocating for anything too revolutionary. I think the change could have been carried through without much of a fuss if the right people had been in favor of it. But they weren't and ultimately carried the day.
Unfortunately, we live in a world where we can't do A/B testing on what the outcome of Hearn's suggestions being implemented would have been but we don't seem to be doing too badly at the moment. Either way, it's always risky to bet against the honey-badger.