Native Arabic speaker here. Can confirm.

Although it’s not always as easy as it sounds because many words and expressions from back then aren’t relevant/in-use. Also, reading poetry from the pre-Islamic era is significantly more difficult. We still have volumes of literature (specifically poetry) that predate Islam.

The reason the language is so resilient is the use of the Quran as a reference (all translations of it aren’t considered Quran, but commentaries), and the fact we have institutes dedicated to maintaining and standardising the language as part of the Arab League (ALECSO and its subsidiaries, etc). Also, all nations of the Arab League enforce a rule that stipulates that the governments, newspapers, media/TV, and academia are required to use Modern Standard Arabic for official business across all member countries.

Regional dialects are kind of hard to understand at first, but generally speaking, we do understand each other regardless of the dialect if the two parties communicating work at it long enough. The hardest dialects to understand are from them pesky Moroccans/Tunisians with their French-every-other-word bastardisation of the language, but we figure it out eventually.

Edit: and I’m not bashing Moroccans/Tunisians/Algerians. We love you guys! ❤️

Edit: spelling, grammar