What Makes a Sefer Torah “Unkosher?”
As someone once quipped, “If a Torah scroll is 99% kosher, it is 100% unkosher.”  That is, if even one of the 304,805 letters is missing or defective; if even one of the words is not in the proper order or is misspelled, then the entire Sefer Torah is considered “pasul,” invalid and unfit for reading at synagogue services, until the defects are repaired.
 
The passage below was scanned from one of our Sifre Torah.  Its problems have been highlighted.
Some of the ink in the “aleph” of the word “Yisrael” has flaked off.  The gap cuts completely across the bottom right “leg” of the letter.  Until that letter is fixed, this Torah scroll is pasul. We could fix it but...
Look at the “roof” of the “resh” in sapir and right side of the “tav” of “luchot.”  These are only two examples of where the ink is beginning to flake off.  In the column from which this passage was scanned, there are at least 20 such defects.  All will become disqualifying defects, with the passage of time.
Fixing these defects will cost hundreds of dollars, if not more.  Given the age of the parchment on which this Torah was written, ink does not get absorbed as readily as before.  That means that every year we will find more and more defects, costing us hundreds if not thousands of dollars.
 
Please excuse the analogy, but this is very much like the economic logic of car repair.  A repair may cost only a few hundred dollars, but the owner knows that even more repairs will be necessary in the future.  That being so, it often makes more sense to purchase a new car - or, in our case, a new Sefer Torah.