Yes, that is the modern understanding. Widespread lead exposure had very little / nothing to do with the decline the Roman Empire.

Plus to prove the lead connection you have to discount the centuries of Roman dominance and growth during which lead exposure was common.

This is a much better point. It's not like there were more lead pipes during the decline than the rise.

Wouldn't there necessarily be more lead pipes at the peak and post-peak? Assuming that pipe-building was some non-linear function of dominance, which seems a fair assumption, we would start with 0% pipes at 0% rome and asymptotically close to 100% pipes at the asymptotic 100% peak rome. Is this a bad assumption, or is it basically just pedantry?

It would be more about lead pipes per capita, possibly as rome itself grew wealthier from conquests they added more and more piping, but it would be hard to relate this to any specific moment in time.

Presumably the wealthy elites had their pipes installed first anyways.

there are many ways to account for the fall of the roman empire, and everyone chooses their favorite (usually depending on where their interest bends). for example, it could be explained by the increased usage of mercenaries in the roman army. i like this theory because the fall was brought by losses to renegade forces. it could also be explained by bad leadership.

"Decadence" likely had nothing to do with the Roman Empire's fall. That theory is based essentially on propaganda, designed to absolve them of blame.

The lead pipes had too much calcification, and not much lead would have leached out. But the Romans did use lead acetate as a sweetener, so they were adding lead directly to many (most?) of their meals.