I remember seeing the Columbia in the VAB up on girders being repaired between flights back in 1989, and it was beginnining to show it's age then. Some tiles were brown, and many were missing.
The immense heat and structural pressure the shuttle has to to endure during each and every launch and decent are enormous. Frankly, I think it was just a matter of time before one broke up or blew apart.
And I'll bet a lot of engineers knew this was risky business too.

We have come a long way in developing stronger steels and alloys since the Shuttles were born..
What should have been made and launched instead of these old birds, is likely already back on the drawing boards. And I'm sure the money from Congress will be there too.
It's a shame hindsight is 20/20 vision.