It is wrong to compare the influence Latin had on Greek with the Germanic and Italian cases.The latter's languages were not as strong and confident as Greek and moreover they were culturally inferior and therefore susceptible.Latin was a dominant language after the Greek (which was now the language of the Old and New Testaments)especially in the Orient.

But of course Greek underwent great changes and lost a lot of what it originally was.The objective fact however that makes Greek language a unique case is its unbroken continuity over a span of 4000 years.Modern Greek and its relationship to Ancient Greek cannot be compared to Latin and the Romanic languages (Italian, French, Spanish,Portugeze),as the latter are only daughter languages and offshoots of Latin, not a natural evolution/continuation of it.Not even to old and new German and even more to modern and old English.Eg,the text of Beowulf which is the oldest example of English (from the 9th cent)is unrecognisable to the average English-speaker.All most all words cannot be identified as English, whereas if u present a modern Greek with Thucidides he will recognise 95%of words,though he may not make sense.In a simpler text however of the same period he will be able to deduce some meaning here and there, and that is a span of 2.500 years!In a study it was found that from the Greek version of the Old Testament out of 4.900 words, 2.280 are currently spoken, 2.220 can be understood and only 400 are incomprehensible.Now the Greek speaker of the 5th cent.probably wouldnt be able to understand Myceanean Greek which was his Ancient Greek,but his own language was the healthy continuation of that language;and this is the case of Modern to Ancient.

In terms of vocabulary and structure Greek survived. A lot of things changed however as other people had to learn the language.The greatest loss was 'prosody',namely the melodical pronounciation of the language-proof of how sophisticated the language was.If it intersts anyone I could post a summary of the changes the language underwent,changes observed already by the classical era.