The problem with Arda was the insane price we paid for him. Plus our midfield has a bigger problem with age than our front line does, not to mention that Rakitic and even Iniesta aren't as unbenchable as MSN. When it comes to midfield, we probably do need a younger signing (i.e. Pogba). When it comes to the front line, it's not time for that yet. A young midfield signing would not need to wait 5 years to make the starting XI (especially if it's someone like Pogba, who'd get into the starting XI immediately). A young forward signing would.
Different needs for different situations. We need a long term starter at RB, CB and midfield to replace Alves and eventually Masch and Iniesta. For the front line all we need atm is someone who can rest MSN. No use paying 70m for a young superstar forward to sit on the bench for 3-5 seasons and be sold for half the price he was bought for. Especially when there are other parts of the team that are aging. A cheap older backup (key word there is cheap so no Gameiro for 40m) would do just fine.