Salam fellows,
i think the reason a new prophet and an updated 'shariat' was sent after some interval was because the human mind was evolving. At each stage it needed a somewhat 'unique way of life' that was practical at that time. So as the people's mind were evolving ,the religion was also evolving. And at the time of Prophet Muhammad (PBUH) this evolution reached its acme and came to a final standard religion or shariat.
"This day i have completed for you your faith and completed my blessings upon you and chosen for you islam as your religion(way of life)." (Surah Almaida5:3)
As the above verse from Quran indicates, the religion was towards its way of completion from the time of Prophet Adam to Prophet Muhammad (PBUH). Even if Christianity(the teachings of Christ) had not been corrupted still a prophet would have been needed to complete the evolution of Shariat. A proof is that Jesus didn't know his teachings would be corrupted then why did he inform about coming of another prophet. Same with Moses: he didn't know his teachings would be modified but he too informed about coming of another Prophet because they knew that the Shariat they were giving was only practical and suitable for that time. For the future, a new Shariat was needed and that was revealed to Muhammad(PBUH).
