The worst I can imagine with the "DLC v. Modding competition" is a situation like that of Cities: Skylines. And with them, there was a healthy mix of good DLCs (After Dark, Natural disasters, mass transit), traces of their content entering the main game, and a noteworthy modding community (Maps/building assets comparable to planet/part mods). With KSP, I imagine big mods (on the scale of USI Kolonization, To Boldly Go, or KSP Interstellar) being able to become DLC. However, with more time and skill available to T2 than the mod devs (not to insult Roverdude or anyone), plus access to the full KSP code, any DLC could easily provide much more than even those mods mentioned, including proper integration of certain plugins and continued support (no need to update ModuleManager or Kpernicus with every update, or worry about the mod being discontinued). The community would still have the ability to produce mods such as individual planet/part/visual packs, but the big ones could be done as the new DLCs that add perhaps much more than any one modder could accomplish.
If anything, don't try and panic about potential C&D letters to a modding community much bigger than even GTA, or a modding system already hardened into the game code getting removed.