You bring up a good point. I've given this some thought. On its face I think it's a reasonable position: if Putin can't have Ukraine, nobody can (not even Ukraine). This is more believable when you consider that oil and gas reserves have been found in Ukraine but it lacks the capital to develop them. Shell was in Crimea prior to the annexation.
But this theory has a few problems.
1. The cost of wrecking Ukraine has been really high.
2. Ukraine was earning a significant amount of money (ie billions) from carrying Russian natural gas to Western Europe through pipelines. Russia has since built other pipelines to avoid this problem but I believe they were still 1-2 years away from completely bypassing Ukraine;
3. Arguably Russiaa can do more damage outside or Ukraine than in it. Russia has been stirring up trouble and supplying separatists in the Donbass since at least 2014. In some ways annexing the Donbass would solve a problem for Ukraine as they'd no longer have this divisive element within their borders. It's arguably more of a problem for Ukraine for the Donbass to be part of Ukraine and this required no further intervention from Russia and certainly no invasion.
It is absolutely a loss-loss situation for Russia and Ukraine. However, as long as the West maintains sanctions on Russia and provides a substantial support (both economic and military) to Ukraine, then Russia will not be able to sustain its war effort and pressure on Ukraine.
Europe is finally, for real (albeit slowly and painfully), moving away from the Rusian fossil fuels this time. It will take some time, but it's a tectonic shift.
But this theory has a few problems.
1. The cost of wrecking Ukraine has been really high.
2. Ukraine was earning a significant amount of money (ie billions) from carrying Russian natural gas to Western Europe through pipelines. Russia has since built other pipelines to avoid this problem but I believe they were still 1-2 years away from completely bypassing Ukraine;
3. Arguably Russiaa can do more damage outside or Ukraine than in it. Russia has been stirring up trouble and supplying separatists in the Donbass since at least 2014. In some ways annexing the Donbass would solve a problem for Ukraine as they'd no longer have this divisive element within their borders. It's arguably more of a problem for Ukraine for the Donbass to be part of Ukraine and this required no further intervention from Russia and certainly no invasion.