That rationale was always illusory. It is government, and only government, that confers and perpetuates monopoly power by erecting and maintaining barriers to market entry, ordinarily in response to political pressures from special interests.
Resourceful entrepreneurs struggling to carve out market niches provide the propulsion for growth.
Nowhere is that more evident than in the computer software industry. It's easy to understand the animus that so many politicians and academics have for the free market. They just don't get it. They've never held real jobs. They've never had to offer anything of genuine value in return for their salaries.