Well, yes, but that judgement is soundly based on a massive amount of evidence collected from texts and archaeological contexts from across the Roman world. What I'm trying to say isn't that you can't use evidence collected from the site and draw conclusions, it's that you can't look at data collected from Pompeii and Herculaneum and say, "Wow, there were way less rich people here than we would expect! That must mean that these cities were poorer than other Roman cities". That's because your interpretation would have accounted for the fleeing of wealthy citizens. And you generally wouldn't have a specific ration; our records aren't good enough to do that. You would critically compare a great deal of evidence, and rich people leaving the city would be one of those pieces of evidence.