I assume there were two reasons that HSBC changed to UnionPay.
1) It is cheaper for them. They need to economise everywhere in their business as they keep losing a bundle on investment banking.
2) Beijing said if you want to continue to do business in HK here is our system and you will use it. We shall give you a really good deal on it.
As a business strategy it makes sense. As a tactical improvement to clients it is a joke.
Mysteriously in Africa there are many banks that accept UnionPay. I guess it is so that the hundreds of thousands of engineers, bankers, developers and assorted hangers on can get a wad out of an ATM.
Go with the flow.