Maybe so, and I've not doubted his quality. I just don't think he strengthens an area where you were weak. It strengthens an area you were already strong in, and ignores an area or two where you aren't as strong. As evidenced by Bendtner now being your back-up striker. And Mertesacker, with his turning circle of a medium sized oil tanker being first choice centre half. Kind of like us ignoring centre midfiled for the last 78 years.
Again, fantastic player, and a statement of intent and proof of a willingness to spend cash, but not a necessity buy in my opinion. He'll improve you, but a striker and a centre half for £40mil the pair may have improved you more.