In this company, the old software is a sacred cow.
sacred cow
idiomSomething which cannot be tampered with, or criticized, for fear of public outcry. A person, institution, belief system, etc. which, for no reason other than the demands of established social etiquette or popular opinion, should be accorded respect or reverence, and not touched, handled or examined too closely.
Origin
disputed originOriginally American English (early 1900s), a reference to the elevated place of cows in Hinduism.
The manager wanted to change the schedule, but the tradition of Friday meetings is a sacred cow that nobody dares to challenge.
Although the department's outdated filing system hindered efficiency, it remained a sacred cow that the senior staff refused to modernize or even discuss.