Do you think he might not see acquiring more Arsenal shares as less profitable than some other investment venture? It isn't going to bring him more control so the decision will be a financial one. If he wanted Usmanov 's shares he could have bought them for less any time over the last seven years. He hasn't and I don't think he wants to. Even if it were to happen there is no guarantee they will be as profitable in the future and Kroenke would be facilitating increased competition for that profit by buying those shares.
Assuming you're referring to his purchase of shares after buying out Moshiri, I think given how negligible his increase in shareholding was, there's very little we can take from it and certainly can't use it as a basis to speculate that he's either looking to hold or sell.
There are so many potential reasons for him to purchase the shares that it essentially means nothing. On the one hand it could be due to a pre-agreement with the prospective buyer of his shareholding or simply because he's confident share price will rise before he looks to sell. On the other hand it could be a case of him gradually increasing his total shareholding for long term investment purposes or just something as simple as his investment/management company in charge of his portfolio doing their job based on market sentiment.