Where in Scripture is it revealed that we should expect an indefinite time gap between weeks 69 and 70?
This cannot be obtained from a straightforward reading of the text.
The gap is what can be called a necessary inference. As with many great truths of scripture, it is not based on one single, obvious text but the cumulative evidence of many verses, scattered here and there, forming a mystery that must be searched out.
What is clear and, as you say, “straightforward”, is that scripture puts the last half of Daniel’s 70th week as arriving at “the consummation / the end”. This end of the age, reaching to the end of the final tribulation, the destruction of the “beast” (Dan 7:11) / “little horn” (Dan 7:8: 8:9) / “vile person” (Dan 11:21) / “wilful king” (Dan 11:36), the deliverance of Daniel’s people, and the resurrection of the righteous dead (Dan 12:1-2) is the same “end” so prominently in view throughout the book of Daniel. There is great, I would say decisive evidence that the final 3 ½ years of this age is the last half of Daniel’s 70th week.