It depends on what you mean by "need"? I mean yeah they have the numbers but what about the quality?
Lacazette - Looks a good signing but nobody can be 100% sure how that will pan out.
Welbeck - Turd
Giroud - Clearly a back up
Walcott - Speedboat without a captain
Iwobi - Form has fallen off a cliff
Ozil - Lazy turd
None of them are fit to lace Sanchez boots.
That's how me, you and everyone else see it. Wenger and Arsenal are deluded beyond belief and have a different view. Didn't Ozil turn down 250k per week or something like that. They shouldn't be offering anything close to that. Wenger loves Giroud and couldn't be more excited about him staying. He will start games. Welbeck has been starting games as well. Iwobi has talent and is definitely a part of Wenger's plans.
Wenger is thinking with those 6 players he's fine, to him the squad is "perfect."
Don't forget Arsenal are not ambitious enough to improve as much as they need to. They will just settle with what they have.