Is it permissible for employees who commute daily from Jeddah or Makkah to the Shu'aiba area, which is about 100 km away, to shorten and combine their Dhuhr and Asr prayers for a period exceeding 25 years, and what is the ruling if they do not do so?

The distance you travel daily to your work is considered a qasr distance (a distance that permits shortening prayers). It is permissible for you and your colleagues to shorten the Dhuhr and Asr prayers and combine them at work and on the way back, until you return to your place of residence, even if this occurs daily. Not combining the prayers is better because it is only permissible, whereas shortening is a Sunnah and it is better to perform it. For whoever omits it, their prayer is valid and there is no sin upon them.