So your view is that whatever pronunciation exists in any dialect will be considered standard by speakers of that dialect, unless that pronunciation causes ambiguity with other letters?I highly doubt that [g] is not perceived as "proper MSA" in Egypt; otherwise, it would not be used by so many educated Egyptians. Similarly, [ʒ] is the standard pronunciation used by Palestinians, and it is by no means considered incorrect. As a matter of fact, [dʒ] sounds dialectal to most Palestinians, and downright laughable to some.
So I disagree with your insinuation that [dʒ] is considered the correct pronunciation by the majority of Arabs everywhere. That's simply not true.
Your argument is not valid. My guess would be that the reason only [q] and [D] are accepted as standard pronunciations of ق and ض, respectively, is that the other pronunciations found in some dialects could be confused with other phonemes found in MSA (the same applies to ث and ذ, for example). ج is truly unique in that all 4 pronunciations found in the Arab world are allophones of the same phoneme. Thus, whether you pronounce جزر "ʒazar," "dʒazar," "gazar," or "ɟazar" (with a voiced palatal plosive), there is no confusion or ambiguity, whereas قُم could be confused with أُم if the ق is pronounced as a glottal stop, and ثار with سار if the ث is pronounced as a س, etc.
So I don't think it has anything to do with تجويد. It simply wouldn't be practical to allow those other pronunciations in MSA, whereas allowing the different allophones of ج presents no problems.
Also, can you think of any other example of a letter whose مخرج in MSA differs from its مخرج in Quranic recitation?
Anyway, here's an excerpt from a review of a book on linguistics by a Lebanese scholar named نادر سراج:
وقدم عرضا استشهد فيه بباحثين وكتاب قدامى ومحدثين وتحدث فيه مثلا عن الجيم في توصيف كل من سيبويه (183 للهجرة) وابن جني (393 للهجرة) عطفا على آخرين. وتحدث عن الجيم معطشة وشامية كما وردت عند رفاعة الطهطاوي في "تخليص الإبريز في تلخيص باريز" (1834) ثم عن الجيم الفصيحة عند الدكتور إبراهيم أنيس في "الأصوات اللغوية" والذي انتهى إلى أن للجيم من الناحية الصوتية ثلاثة أنواع هي.. "شديدة خالصة الشدة وتلك هي الجيم المصرية ومزدوجة من الشدة والرخاوة فيها من الصفتين معا وتلك هي المسماة بالفصيحة وأخيرا تلك الجيم الرخوة الخالصة الرخاوة وهي الجيم الشامية. ومخرج النوعين الأخيرين من وسط الحنك..."
Of course he's quoting إبراهيم أنيس, who was Egyptian.