I'm no engineer, nor do I have any particular knowledge on the matter. But what orbital conditions could possibly stress an external tank more than that of the launch? And no it is not a rhetorical question, I'd genuinely like to know. Micro-meteorites? Containing an atmosphere?
But that is a weak argument at best for getting rid of NASA, arguing what they have not
The stronger argument by far is what they have
done. They have made it much more expensive(even with inflation adjusted) to put stuff into orbit. That is government waste at its finest.