There is always competition under a system of capitalism.
It is hard to predict new and emerging technologies and how it affects the way we do things.
The pony express was replaced by faster forms of transportation. Trains and other vehicles worked out great for the postal service. The postal service employed millions of people directly and indirectly too.
As mentioned above, the United Parcel Service (UPS) was started over 100 years ago as a delivery service. That business took off especially as gross consumerism ramped up.
While the postal service could handle billions of pieces of mail and parcels annually, larger packages and faster delivery times required something different. A demand and convenience private companies were more than wiling to fulfill under a system of capitalism.
IMO, long before the internet and mail, personal computers should have been viewed as the technology that would drastically alter the way we collect and store information (data) but also how we communicate.
Email was invented back in the mid-1980s. Same goes for desktop publishing and digital storage.
The analog world i.e. everything that required pen and paper; envelopes, stamps and book-binding would be changed a few short years later.
I believe postal services will always be around. Just like other jobs with AI impending, the demand will shrink.
Therein lies the blessing and curse of technology especially under a system of capitalism.๐