Correction, those aircraft that were shot down PRIOR to MH17 were shot down with 'standard' surface to air missiles that had a top ceiling of just above 20,000ft. MH017 was unfortunately the incident that proved the rebels had been equipped or had captured more capable weaponary.
Once again, airliners are flying every minute of the day over areas of the globe where aircraft are being 'shot down' at low to medium levels. In these cases the airlines will still transit as there is no evidence of BAK style weapons being deployed there.