It feels to me like you are misunderstanding that particular point.
To elaborate further - these things are not developed in isolation. That is to say if we pursue one thing, we do so by not pursuing something else. Similarly, if we chose to not pursue that one thing, we do so in favor of something else. All of these options can enhance the sandbox experience and must be prioritized according to their assumed costs and benefits.
I don't want people to get the impression that all is set in stone because I keep going back to this point (to clarify it). It is only my current impressions that the cost and benefits of this aspect are worse than those of other priorities. Maybe a good solution will be identified that can drastically reduce costs. Maybe other people will view the value higher than I do.
To get a bit more back on topic - A question to those that are interested in defensive keep fights. Would you feel that the current offensive set up would offer an enjoyable defensive experience to you? That is to say - no control over your troops, no reinforcements for your side and generally a mission that you are quite likely to lose (or, well, are intended to lose - since another challenge may be players exploiting the terrain to simply 1on1 200 bots and basically mean that no siege has to be lost anymore... encouraging that very approach). If not, how do you imagine it should work?