When Peter Adderton started Boost Mobile in 2000, his goal was providing competitive wireless service to struggling consumers, a market he believed large carriers were neglecting and, on occasion, exploiting.

Nearly two decades later, he worries that T-Mobile’s $26.5 billion offer for Sprint, which now owns Boost in the U.S., is the beginning of the end for the brand as well as for MetroPCS, a key rival in the market for pre-paid services often used by people who lack the credit needed for traditional mobile services. That would mean escalating prices for some 30 million Americans who can least afford them.