You whole assumption rests on your 'personal impression'. You simply can't know what other people in the open source world think or presume everyone agrees with you. To claim 'one voice' seems a rather large stretch. There are tons of Ubuntu users and others in the ecosystem who may think differently.

What makes you so sure Mir would not have some advantages? It seems impossible to say either way. For all we know it could offer benefits and choice to the ecosystem. Or die because users would reject it.

Open source has always been about diversity and choice. There is now an unfortunate trend of some commentators misusing words like 'community' to push their own agendas or trying to pass off personal opinion as a widely held fact. This basically challenges open source diversity.