They are clearly the best 2 fighters of this generation, spanning 10-15years. One of them won fighter of the decade, the other was in contention for it. There have only been 11 decades of "modern" boxing. You can only do so much in an era. If you extract 2 top fighters per decade and even make some of the worst case assumptions such as this era somehow being the weakest of all 11 decades, they'd still be sitting at 22-24ish. Even then, such an assumption is massively subjective.
I like your rationale. Top 30 for both is very fair, and quite an accomplishment to be ranked that highly in the history of boxing. I like it.
When questioned as to why I rank both in the top 20-30 ever, I simply say that there is only 120 years of boxers to choose from. They dominated for 15 years. When looking at ratios alone it is a fair statement. Arguments could be made for the contrary but then it would just be some sort of over-glorifying of history. The sport changes.
Your not taking into account that old time fighters get made out to be 10 times better than they actually were
Exactly. When looking at things from an objective standpoint, even if you take some of the worst scenarios they'd still be ranked on the lower end of 25-30. One of the more objective resources available to us this day is the boxrec database. They recently posted a bouts-per-year measure and I took the liberty of converting it to a chart. http://i.imgur.com/IvTX3p3.jpg I think it's pretty safe to say that the higher total number of bouts can only serve to bolster an era's competitiveness. Now even if we were to hold each era independently, by factoring out precedence and ease of access to information, I'd say the current era really isn't too bad.