M&As are not inherently good or bad for shoppers.
Bad for shoppers: Huge MSC buys small MSC. Huge MSC cuts fees drastically and, unlike small MSC, treats shoppers like commodity. The larger and more powerful Huge MSC gets, the more likely other MSCs try to compete by slashing fees.
Good for shoppers: MSC1 is poorly run and has trouble paying its shoppers. MSC2 acquires MSC1 (for whatever reasons) and pays MSC1's shoppers for shops performed.