Central banks usually buy back government bonds to combat deflation and facilitate economic growth. This policy is commonly called "quantitative easing" or QE. After the arguable success of QE during 2008-2009 recession in the United States, many other countries choose to adapt this monetary policy.
It is very unlikely that the Bank of Japan will keep this policy until 2026 because at some point inflation will pick up and they will have to stop buying bonds and perhaps even raise interest rates. In the US, UK, and EU the bond-buying quantitative easing programs haven't lasted longer than 5 years and they are usually closely monitored and can be stopped at any time.
When the central bank buys government bonds, they stay on its balance sheet until maturity. Normally, central banks are happy to keep them as a convenient monetary tool. They can sell the bonds back to reduce inflation. Whether the government debt on the central bank's balance sheet can "cancel out" depends on the countries laws.