Basically yeah, multiple people left all at once, including head writer Russell T Davies. Pretty sure it was amicable. Stephen Moffat took over as head writer for Season 5 (and onwards). Moffat wrote a few of the episodes in the RTD era, probably most noticeably The Empty Child/The Doctor Dances and Blink. But that's why the ending of The End Of Time was particularly sentimental, because it was the end of an era beyond Tennant's run as the Doctor. Some people (myself included) didn't like that ending for a number of reasons, others think it was one of the most emotional moments of the new series to date.
Fortunately, outside of a vocal minority most people regard Moffat very highly, so rest assured the show's in good hands going forward!
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James - Board 8's Resident Warm And Safe, Slipper-Wearing User
& The Cream of Porcupine Tree Fanboyism