Time spent on it is time taken away from AH. If you want updated planes, new planes,new gvs, new content....
That is very narrow and one-dimensional thinking.
Any plane he creates for WO:P could be merged back into AH.
Any boat or vehicle, or models, or textures he creates for WO:P could be merged back into AH.
It's the same technology base layer. Hitech is smart enough to plan for cross pollination and mutual support between products. Think synergy, not zero-sum.
More importantly it gives him a testbed where he has more design freedom to try different things without having to do so on the flagship product. It would be dangerous to try things just for grins on AH. Worst case: it fails on some side product and he learns from it. AH is undisturbed. Best case: some thing proves useful and he can migrate those particular improvements back to the AH code base after it has been wrung out and real-world tested. AH inherits the improvements with much less risk.
And most importantly, if HTC can expand and diversify revenue streams, it makes the company more stable and increases the odds AH will stay around. AH will not outlast HTC. So anything that makes HTC stronger, makes AH stronger. Right now, there is a single point of failure just waiting to happen. Every egg is in one basket with no margin of error.
Not to mention additional revenue can fund more 3d art, advertising, etc. ...