Funny you say that... Russia has literally sold a billion or two worth of stolen grain already, much of it straight out of storage silos in occupied Ukraine. They also were selling off metal stocks seized in Mariupol and other places. Similarly, we know they took over many mines and are now operating them. Also in Mariupol and other parts or occupied Ukraine lots of Russians have moved into stolen apartments and houses – the original owners often being dead. They have formal processes to take over property without compensation.

You don't have to speculate on this. Russian TV talks about how Ukrainian resources and land are going to strengthen the Russian economy. They know they're stealing it all. 

 Sure, there are some spoils of war. And some leaders will brag about victories and gains to appeal to their citizens self interests. And attempt to humiliate and demoralise the enemy.

But that doesn't make theft the motive for the war.