You answered your own question with the bolded part.
Yes, Steven may be our best player at the moment, but *at the moment*, the side is absolutely terrible. What we need, is a player that could be our best (or, at least very good) *later*, when we're challenging and will actually benefit from their talent.
In a previous post I outlined why I believe that Steven will not be as good when our side is at its peak (the nuts and bolts is that, given his lack of skills, he is largely a physical player, and such qualities drop off with age, which will be 28+ when we're in our window). Combine that with the fact that we'd get VERY good compensation for him (otherwise he wouldn't be traded, of course), and we currently have a very solid recruiting team, I'm of the belief that if we were to trade Steven, the player we end up drafting and developing will be more influential to our success than Steven when we're challenging.