A naturally earned monopoly is perfectly acceptable, because it is the result of market forces.
The DOJ only opposes mergers and acquisitions which would substantially alter market conditions to increase prices faced by consumers in a way that market forces would not normally support. For example--Verizon and AT&T could never merge, because their combined market power would effectively let them set whatever price they wanted for nationwide cell+data plans. On the other hand--Sprint and TMobile could merge, because without the merger both companies are in danger of failing but as a result of a merger could theoretically compete with Verizon and AT&T.
I'm not seeing the distinction between "market forces" and whatever other forces you're talking about. If Verizon and AT&T merged, how would that not be the result of market forces? Of course, that's a bad example, because there's already not a whole lot of market forces in telecoms since the government controls access to the airwaves.
The DOJ only opposes mergers and acquisitions which would substantially alter market conditions to increase prices faced by consumers in a way that market forces would not normally support. For example--Verizon and AT&T could never merge, because their combined market power would effectively let them set whatever price they wanted for nationwide cell+data plans. On the other hand--Sprint and TMobile could merge, because without the merger both companies are in danger of failing but as a result of a merger could theoretically compete with Verizon and AT&T.