Of course the WTS does not start its "70 years" with the destruction of Jerusalem but with the exodus of some Jews into Egypt, following Gedaliah's murder.
I say that the exodus took place 4 years after Jerusalem was destroyed. In any case, the WTS demands that the land was devoid of people, and this did not occur immediately after Jerusalem's destruction, with the Babylonians installing an administration there under Gedaliah. It is most likely that when the Babylonians invaded the land during Nebucahdnezzar's 23rd year that this was in response to Gedaliah's murder.
At the other end of the WTS's "70 years", they wait until the people were in the land, had all settled into their towns and villages, traipsed to Jerusalem, and built their altar before they end the 70 years.
On top of that, when Ezra later wrote 2 Chronicles, he said that the 70 years ended when the Persians defeated Babylon, not when people assembled at the temple site.
And further, no one knows the year when those exiles from Babylon made their journey - dates from 538 to 535 are given. The WTS cannot prove its date.
And further, archaeological evidence shows that the land remained occupied - the Babylonians needed the land to be harvested for food, as the Bible indicates.