While in 50 years it might not be a great building anymore, it will still be a fantastic pile of rubble. Basically landfill, but it can be reused as gravel for new building projects, too.

The concrete won't release the CO2 when it breaks down, since the quicklime in the cement reacts with CO2 to form limestone. The catch is that quicklime is mainly produced by heating limestone to release CO2, so making extra concrete won't result in net carbon capture. But if the concrete was going to be produced anyway, I suppose it's better to have it absorb the CO2 sooner rather than later.

Roman concrete structures still exist after 2000 years. If you want to "hide" the CO2 somehow, then concrete doesn't seem like a bad idea.