The purchase price of an elevator is a fraction of what it costs to own one over its service life. When a system is locked to a single manufacturer's service network, every maintenance visit, spare part and software update is priced without competition.
Non-proprietary systems keep that market open. Because any qualified technician can service the equipment, you retain leverage on price and response time for the entire life of the building.
The lesson for developers is simple: evaluate the fifteen-year cost, not the day-one quote.