The merger did precisely what they wanted it to do:
Give a single player a dominant market-share. This single player will then engage in price-flooring practices until they've snatched up the largest market-share. They will then enforce their gradual fee and upselling strategies.
This is no different to any other market where a monopoly exists. Internet companies like to claim they aren't monopolies by saying "just visit another site for their services" but entrenchment of the market is exactly what makes them monopolies.
If a large-enough competitor came along (with big funding backing it), you'd soon see Upwork change tack and go back to price-flooring.