Why wouldn't you get both? I don't understand how this managed to become an either or scenario.
I guess you're not very familiar with playing position 10 support. I consider having both Euls and Blink on a support a straight up stomp.
Why is this a point of contention? Nobody *needs* to have anything. But some items are core for a reason.
A core item is one that is effectively a necessity because it's straight up better than every possible alternative. Aghs on Warlock is core. Blink on Magnus is core.
Blink on WW is no more core than Blink on SF is core. Blink SF enables a very specific kind of initiation/positioning based gameplay, but he has other build paths that don't need Blink at all. Similarly, Drow, Troll and Sniper have a choice between "gotta go fast" builds (MoM, yasha) or "look at me I'm a ranged anti-mage" (Blink), all depending on team comps and matchups.
This is the space I think WW exists in. Blink is very nice to have on her like it is nice to have on CM, but they can both cast their spells without it because of their extremely long range. And, it's not always a given that the enemy team is stupid enough to bunch together for 5 man curses. In that scenario your Blink would've been better off as a flexible Force Staff or Euls.
You're treating her as Sand King when she is really closer to CM or Phoenix.