Question: Why weren't there any temples?
The 12 tribes of Israel were already scattered. The Northern tribes got the ass beat first by the Assyrian empire. They were done. The souther tribes (Levi, Benjamin & Judah) remained in Jerusalem till 70 ad. There were no temples left...peep the book I posted above. You can see the invaders destroying and pillaging everything from Judea.
Question: Wouldnt we have any trace left?
TMH said he would keep a remnant...the punishment for falling away from the law was that Israelites would get destroyed...lose they culture...they language, lose their heritage...but for a limited time. TMH will restore them...so according to the text they wont be FULLY assimilated, but many will be.
Isaiah 10:21-22
21 The remnant shall return, even the remnant of Jacob, unto the mighty God.
22 For though thy people Israel be as the sand of the sea, yet a remnant of them shall return: the consumption decreed shall overflow with righteousness.