johnamos
Jeremiah's focus concerning the 70 years was that those 70 years would be the period that Babylon would be a world power and during such time the nations would have to serve Babylon as such. Serving Babylon was not dependent on being in captivity or desolate. The serving started as a result of Babylon becoming a world power and it ended when their time as such a power ended.
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Your interpretation of the 70 years that it is only one of Babylonian servitude is wrong and does not agree with Jeremiah and the 70 textual corpus. The 70 years was a period of exile in Babylon, a period of the land being desolated and a period of servitude to Babylon as shown by Jer. 25:11-12.