BBC Four & CBBC to close

[L1]

However, BBC Parliament were unnecessary channel because it can be easily simulcast with BBC News channel when Parliament was not in session. So, why not merge both channels instead with World News?

Do you mean merge Parliament with combined NC/World News? Rolling coverage of the UK parliament(s) would never work on the domestic NC let alone to an international audience surely? It seems the BBC are quite sensibly picking their battles as closing Parliament as a channel would save a miniscule amount especially now all bespoke programmes have gone.