The thing is, if you are going to be close enough to strike something with surprise, you will be too close to rely on those 2000 men to protect you. Most places that you want to strike with an army, will have very sizable forces already there protecting it and those forces could make it out to overwhelm 2000, 4000 or even 6000 men, depending on how far away the invasion point is. If you are far enough away that an army won't be able to get to you in time to stop you from bringing over your army, then you are too far away to have surprise and they will be prepared for you.
It's certainly an advantage to be able to transport men that way, but it's not something overwhelming or broken.
Surprise is relative. IF an army of 2000 pops up next to (lets say a hours ride) a keep of a couple of hundred (at best), they are going to surprise that keep and overwhelm it keep pretty quickly. They can use that keep as base of operations moving forward. Have an army of 10,000 in a few days and overwhelm a kingdom not prepared for a large scale battle. I don't know about who you, but have an entire kingdom's army appear within your borders in 5 days when it would have taken weeks or months to get there normally seems like a big surprise.
I don't think the idea that demiplane creates a vast plane broken or overwhelming form a typically player perspective, but for me it is from a world building perspective. I also think such power should be reserved for the gods.