This is what I like about this site, people willing to give there time and effort with an unbiased slant in response to questions/statements, so thank you.
I'm not trying to convince anyone about my take on things, it's more about establishing in my own mind that my strategy might have some merit, so to just counterpoint some of your points.
A fallback position for me if Witts goes down ( a big man coming back off an ACL certainly has risk attached) then I can drop say a Sicily type down to a performing rookie and get one of the big dogs as R1 (I'm happy to use Preuss in this role if he's rolling ok) so 2 trades for the Sicily solution as against 1 for Preuss. Of course I don't have to use Sicily if he's performing well, there will probably be an underperforming premo in my team anyway.
I have Mitchell, Walsh, Petracca, Jelly and Neale in the mids with Lloyd back and Dunkley forward. There's certainly a cost in not having that R1 as a C option for those rounds until I can upgrade.
Coleman is an interesting one and his position in my team is completely reliant on him maintaining the role that he had for the last 3 rounds of the year where he averaged 85, I'd be stoked if he kept that up because, as you said, I'd only need a maxxed out rookie to trade both to a top defender, whereas I'd need 3 rookies at least to achieve the same outcome. It's exactly the same line of thought that I'm using for Rioli, in 5 of his last 6 games he averaged 91 (excluding a 54 because of 7 clangers). The combined benefit of those 2 players over rookies is possibly 65 points/round for around the same cash generation as well as using 2-3 less trades in turning them into premo defenders, the other advantage that those 2 provide is the DPP flexibility in tandem with De Koning, Skinner and Kelly. Cogs to me is a no brainer.
The combination of Cogs, Rioli and Coleman replacing rookies could be around 100 pts/rnd somewhat compensating for my weak ruckline, but come upgrade time (and with a bit of luck) I could add a premo/week using just 2 trades/week for 4 weeks hopefully putting me ahead of the pack and only needing 5 more upgrades to complete my team.
I'm relying on 5 speculative picks to achieve averages that they have either proven to have done in the past or should do given the roles that they have been pegged for. I actually feel that I'm taking more risk with players like Sicily, Maynard, Heeney and Thomas (who aren't set in stone as picks yet)