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But we all know what happens if we spam the network to fill up blocks, this doesn't prove anything we don't already know. It doesn't remotely simulate the real world where real forces like innovation, adaptation and potentially fees all play a part in how things progress.

You don't know the impact it will have on decentralization. One of the reasons for the limit is protecting decentralization, do you think it hold if the UTXO set get to several GB ?

It will simulate innovation and adaptation, if the experience last for several months, people will need to take steps now to fix the problem.
What step will they take ? Will they leave Bitcoin and use centralized credit systems (intra coinbase transactions) ? Will they continue to use bitcoin only with higher fees ?
Will payment hubs be developped and incorporated to wallet software ? (please merge CLTV !)
Will they go to an alt coin ?

I don't think it is clear what will happen. But we can find out very easily with such experiment. If the result is one where the whole ecosystem disagree (and by that I mean the tech community also), then at least we will know how to evolve Bitcoin instead of staying in such divided opinions.

The problem is not 1MB versus 20MB, the problem is that we can experience a split of currency if the ecosystem continues to disagree.