If he's the sort of player that doesn't put in any effort when he doesn't get his own way then i'm glad we didn't buy him, he's under contract and should give 100% every game for Arsenal.
As it is, i don't think he's that sort of player
I understand where you are saying but, coming from a place of always having had a contract for any position I have held since university, I think football is the only realm in which people think this way. If you are not a footballer, want to terminate your contract with your employer (to pursue a better position elsewhere, perhaps for a better compensation package or more professional prestige, or both), with willingness to except outlined penalties and/or ensure they receive reasonable compensation for the loss of talent, but they refuse and threaten severe legal action to prevent it, forcing you to stay and see out the contract, you would be livid and your morale and motivation would be significantly impacted. Thankfully, that rarely happens outside of football—often because contracts can be difficult to enforce but more often than not because the employer realises the moment you've decided to go you've immediately become less valuable (read productive) to them and they need to mitigate the loss and transition as best they can. The few times something similar to this Sanchez situation has occurred in the non-footballing professional world has been down to a star talent (usually in executive, sales, or R&D) wanting to go to a competing entity or to start a competing enterprise. Ironically, though, in many of those cases the original employer reacted in the same way Arsenal has in this case and gone about shackling (or worse, professionally destroying) said talent which, in the end, usually leads to several bad states:
1) the talent was no longer productive despite staying on
2) morale of current talent was depressed as they had now seen they have no options apart from staying themselves, even if they were not happy
3) talent acquisition became much more difficult as outside observers saw that once you are in "you are in, until the contract is up"
4) potential investors are put off as they see the company struggling to effectively deal with unhappy talent, maintain healthy morale, and attract the talent necessary to sustain any current success and potential grow the business
5) overall production is significantly impacted by the combination of the above states, causing a downward feedback cycle where each bad state worsens causing the others to worsen, as well
The outcomes shown above are one of the reasons most successful companies do their best not handle these types of situations the way Arsenal handled Sanchez, if at all possible.
I think you could easily apply all of those negative outcomes to this situation, as well. Perhaps we an additional state of having now locked in an unproductive talent, rumoured not to be particularly popular with the other employees, during a operational period crucial to the continued success of the enterprise, with the all most certain resolution of losing said employee at contract end with no compensation and little in the way of a transition plan (not to mention increasing the chances of negatively impacted production during that time).
All in all, it was a horrible botched endeavour on Arsenal's part and has a very high chance of completely derailing a season that is already started badly.