My late ex-wife was an engineer at KSC for 17 years, and worked on Station and on Shuttle. She hadn't been working for NASA for several years when Columbia happened, and her analysis from outside was that it was *not* the insulation. She repeated, many times, that Shuttles had lost that much insulation before and come down fine.
She believed that the problem was more insidious, and partly due to management. What she, as a materials scientist, said was to point out first, that the Cape is on the Atlantic Ocean, and there's a constant salt water content in the air, which happily causes a *lot* of corrosion. (Those of you old folks here might remember what crrome bumpers on cars looked like that lived near the ocean.) In metals, it causes stress corrosion cracking - microcracks that need close inspection by experienced people to find... and which need replacement when observed. Further, the hydraulic lines inside the wings were in that environment, and this was a danger to those lines. If they were to rupture in the stress of a mach 25 maneuver, well, the Shuttle suddently has the controal and aerodynamcs of a mach 25 set of car keys.
She also said that those hydraulic lines were rarely checked, and that she was one of the few who *could* check them, partly because they needed looking at by experienced, knowledgeable people, and partly because she was five foot tall (on a good day) and 105lbs soaking wet... and the space that the hydraulic lines ran through were *very* small and tight, and most folks would have trouble getting their heads in. On top of which, management was getting lazy, letting experienced people (not just her) go, and not hiring replacements, nor demanding all the inspections that the rules demanded.
She figured that's what happened, and in an instant, the hot, flammible hydraulic fluid is all over inside the wings, and it was all over.