First of all, those statistics from Wikipedia may be a bit misleading, depending on your point of view. What they seem to have done is count every word in a 80,000-word dictionary once, regardless of whether the word is very rare or very frequent. Consider the preceding sentences:
-First -of -all, -those +statistics -from -Wikipedia -may -be -a -bit -misleading, +depending -on -your +point -of +view. -What -they -seem -to -have -done -is +count -every -word -in -a -80,000-word +dictionary -once, +regardless -of -whether -the -word -is +very +rare -or +very +frequent.
I've marked the words of Latin or French origin with a plus sign, the others with a minus sign. That's 11 words of Romance origin out of 44 words, so 25 %. Note that regardless is dubious, because it is a word that French had borrowed from a Germanic language. Your average spoken English contains even fewer Romance words. The percentage of Germanic words generally goes up as the Romance percentage goes down.
The reason for this is that English has a very long tail of Romance words, but they are much less frequent on average than the words of Germanic origin. If you take a list of the most common words in English, they will be overwhelmingly Germanic.
As to the replacement of existing words, yes, sometimes they replaced existing Germanic words, but at other times they rather enlarged the vocabulary, especially the various technical and legal terms. The distinction is often difficult to establish, and not seldom meaningless, because many words become less frequent on their own account, so it is not always clear whether this is caused by pressure from a new word or just for no obvious reason.
The Romance words that came to England were mostly of a high register, as in law, rhetoric, philosophy, science, etc. There was the pressure from Latin on the one hand, the lingua franca across Europe, especially in writing; and from 1066 on, there was French, the language of the conquerors, who brought with them various elements of bureaucracy and high culture in French. It is usually the simpler, more frequent words that withstand such pressure the best, whence our continued attachment to words like is, the, a, he, she, do, be, have, etc.