You apparently don't know the difference between 4G and 5G. 5G requires millions of new towers of about 25 feet in height. Higher frequencies can't travel long distances. 5G is going to provide gigabit internet service wirelessly to the home, but on fiber from the many new towers to the backhaul.
There is no new 'business' that is bigger than the implementation of 5G. It's the whole Internet of Things along with FirstNet and many, many new things. It will cost hundreds of billions of dollars to build it out. There will be enough work for anyone with tower expertise to enjoy contracts for years to come.
The old towers will still remain, but primarily for 4G and older technologies.