You're right, they should just roll over and accept Russian domination.
Their comment, of course, was overly simplistic. I'm sure what they meant was "then why are they protesting action that will end the war in Ukraine's favor faster".
If you only care about blindly ending war as soon as possible by any means necessary, you definitely have two major options. Either let the aggressor do whatever they want, or use overwhelming force to utterly destroy them.
Which is your preference?