If you are in a daily lineup, i cant make any sense of what your are saying and the reference of position eligibility seems irrelevant to me. ESPN doesnt force you to have only 2 pf. If you have 2 better pf than Turner then you can roster them as Pf and F. Then again, as mentioned before Turner is also a C so you have to have a better Center than Turner as well. Then again, you got 3 utility spots, that means you have to have a pg a sg and a sf better than Turner and 3 others more. So you actually have to have 9 better players than Turner (could only seem plausible in an 8 team league) and that is if every player in your roster plays every night. If you add the fact that every day you will have people not playing, then everything i have said so far is pointless and Turner is simply worth rostering every time in every format.
Now if you are in a weekly lineup, position eligibility is not irrelevant and your post makes sense. You are saying that if you have about 8-9 better players than Turner in your roster (including a pf, a C and one more forward), you are set on blocks but you are thin on guards for example, then it makes sense to drop him for streaming. If this is indeed what you are saying i would still keep him tight and not even consider dropping. First of all, Turner is a big guy but he is giving you 3s with nice ft %. Also the fact that you are set on the block category doesnt mean you should get rid of him because i 100% guarantee you he will be claimed by all the other 7 owners in your league instantly. The one who gets him will probably be better than you in blocks now! Anyway, the funny thing its that all this conversation isnt about Turner at all. Its about a scenario with a team which has a specific build in a shallow league with specific settings stacked with big men. But you could argue the same thing for Milsap, Griffin and Whiteside, if the were the worst big man in your team does that make them expandables? The point is, that even in this scenario Turner remains undropable. Not "James Harden" undropable but "must own in all formats if healthy by all means" undropable. He is top 50 and you cant find top 50 in WW. Ok sometimes you can, but still, dont you ha a single player that is not top 50 in your team to drop instead?