People seem to have a general distrust about the Indian government and the UPI system. There are certainly a lot of conspiracy theories floating around.
The plain idea of UPI is that it is publicly funded digital infrastructure. The US has networks such as Mastercard, Visa, etc. These are privately run digital infrastructure that facilitate digital payments. UPI is publicly funded digital infrastructure. AFAIK it is legal for private digital infrastructure such as Visa, MC, etc to exist. However, due to the tax payer funded nature of UPI the fees are way lower than private digital infrastructure can offer. This has been the main criticism against it by the private players.
Let's be clear, governments everywhere build roads using taxes and offers them for "free" to the citizens. That doesn't stop private entities from creating private roads for a fee. However, no private entity tries to build road infrastructure as it is not a profitable business. Thats the same situation with digital payments in India due to UPI's existence. That doesn't mean that private businesses cannot offer their own private payment networks with value added services to compete with UPI.
The mindset in the west is very different than the east which leads to a whole lot of unfounded criticism on here.