They certainly have quite an extensive network, but to say that they fly almost everywhere is just not true. If one only travels within the USA or from the USA somewhere else and back the options are fine. If one wants to travel within other regions DL has virtually zero coverage. The only exception to that is the NRT hub that they bought with NW, but even that has its limits as NRT is generally inconvenient for connecting within the region (e.g HKG-NRT-GUM v. HKG-GUM). And SkyTeam definitely is the weakest alliance in terms of coverage and it will get worse with the exit of CO and the 99.99% likely exit of CM.