A lot of people who bring up the topic are talking more about the present than about Roman days. Many of them are looking for some kind of answer or law to explain why societies decline and fall. Some of what they come up with is wrong or has more to do with our own condition than with anything that happened in Rome.
But the "other side" that doesn't want to discuss the question makes similar mistakes: a Boston Globe editorial of a few years back said that it had been proven that lead in Roman crockery brought the empire down, so all talk of decadence was pointless. I hope we can all agree that that's not an especially useful attitude.
Rufus Fears teaches at Oklahoma University, and has recorded lecture series for the Teaching Company. They're interesting and useful, and Fears is certainly conservative, but he's more of a preacher or popularizer than a detail man. Anyone who wants to know more about Greece or Rome, or Winston Churchill or the history of freedom might look into his lectures, though they're certainly not the last word about such topics.
The problem with discussions about why empires rise and fall is that objective and subjective factors are so entwined. Bring in morality and things get yet more complicated. The British empire faced serious anti-imperial sentiment in the 20th century. But the objective fact of two world wars probably did as much to doom the empire as anything else. And if we ask ourselves whether the empire was a good or a bad thing at this or that point in history we get into quite a discussion.
There was a speech that Ronald Reagan gave when he was Gov. about the similarities between the fall of Rome and social trends in the USA. What he pointed to for the fall of Roman unwillingness of young, privileged (liberal) Romans to enter the military and defend Rome, abandonment of traditional morality (acceptance of homosexuality and philosophies of the Cynics, etc.) He attributed part of the downfall to the Romans "outsourcing" their defense to foreigners instead of shouldering the burdens themselves. Eventually it became so expensive to pay others to defend the Empire that taxes became burdensome.
This speech pointed out that Rome enjoyed a kind of "pioneer heritage" of a sort, rose to its zenith, and then declined in its third century. Reagan pointed to the social decay in the US and declared that it threatened to hollow out our ability to protect freedom.
I used to have the speech in hardcopy years ago, but I've since lost it. I've searched online for it a couple of times, but I haven't found it yet. If anyone else has it, it would be a pertinent addition to this thread.