Nobody is disputing the existence of the Islamic Golden Age. We are talking about the intermediate, bloody and chaotic period caused by the Muslim conquests that rapidly spread from the Arabian peninsula and overcame the Persian Empire and most of the Roman Empire. These empires didn’t go away for a “variety of reasons”, they went away because they were violently conquered by invading force in the name of religion.
https://en.m.wikipedia.org/wiki/Early_Muslim_conquests
As for your claims about Alexandria:
> Following the Arab conquest of Egypt in AD 641, the city was plundered and lost its significance before re-emerging in the modern era
https://en.m.wikipedia.org/wiki/Alexandria
Once can also hardly read accounts of the conquest of Alexandria and think “my, what an improvement.” A lot of people died. Parts of the city burned. You are also wrong that the schools were closed prior. That was certainly not the case. Alexandria was a major center of learning up until the conquest.
Carthage was sacked and deliberately destroyed to prevent it from being retaken.
Look, yes, there’s a group of people out there that understates they Islamic Golden Age. But that doesn’t take away from the reality that the early Muslim conquests were absolutely ruinous. People were still writing those “important classical texts” you mention when they were killed and their texts burned. I doubt they would have much appreciated that the conquerors much later translated what survived the flames.