To be less sarcastic: You can minions swarm bases down from as far back as your last tower. Even with minions, a DG fighting at a tower is probably going to beat another DG, and hey, if it's erebus you have the added benefit of being able to flee easily.
And the the thing is, there is a problem if your team doesn't have strong AoE. And when I say strong, I mean "either you have mass charm or something that one shots the enemies."
My suggestion is to make the range interval for all minion affecting items 25 yards. This allows builds that don't have strong AoE to still beat minion swarms, by simply driving off the DG; then the minions lost a lot of benefits and can't be horn of battled. You still have to kill them, but they aren't going to be death for one team and cake for another.
Also, 1800 health on Nightwalkers isn't very hard; they start out with around 800 health, conversion aura pushes them to 1200, Hauberk of life pushes them to 1400, and from there it's just Morale. Combine that with a point in Mist, a point or two in Batswarm, and by level 9 you have 10 1400 health Nightwalkers. Getting a bit more HP is tricky, and I might have overstated 1800, but 1400 is still a ton of HP to take down by any AoE ability. Also, around level 10 you can get platemail of the crusader, which brings nightwalkers up to around 1650 HP. It's weak early game, obviously, but the basic thing is that you can easily keep destroying towers and buildings unless the enemy has strong AoE attacks, and since Erebus can always get away, there is really nothing you can do about it.