Zlatan comes with pros and cons, but the pros definitely outweigh the cons.
I get what you're saying, he's missed some big chances this season, particularly in those frustrating little draws against Stoke, Burnley, etc etc. But it's hard to be annoyed with him when he continuously makes up for it and more in the majority of games.
I was watching the Man City game earlier today, and I do agree with the lads on here; our other 3 of the front 4 simply NEED to chip in more, big time. Sane and Sterling take pressure off Aguero because you always feel one of them will score, but with us, if Zlatan is having a frustrating day, 9 times out of 10 we as a whole do too. It's not a case of Zlatan not scoring every single match, or running the channels or doing anything differently, he just needs a bit of the burden lifted off him from those around him. He leads our line fantastically as a physical target man with exquisite technical ability; his little touches and interplay suits us to a tee when we're on song with Mkhitaryan, Mata and Martial nearby, the trouble is I feel when Mkhitaryan is out of the equation, as we've had to deal with lately, there's nobody like him in our squad. Similarly to Mane and Liverpool. So I definitely would welcome an extra quality addition in attack, Griez would be a dream.
But the answer to your question, I don't think Ibra is an issue, if anything I'd love to continue to build around him, pity we've got him at this stage of his career.