Again, I'd like a link to your monopoly claim. This proposal wouldn't be a monopoly though. People could still pay for a faster or better service, perhaps bundled with a phone or TV package. It's akin to saying the NHS has a monopoly on healthcare in the UK just because it is free at the point of service (in Scotland at least).
Would it damage private providers? Maybe. Would it benefit the well-off disproportionally? Arguably. It's mainly a subsidy for rural voters. My dirt-poor neighbours can already get 150Mb/s because we live in a city - and those who can't afford it can piggy-back off my 50Mb/s.
Yet again I'm defending a policy I don't agree with from a party I won't be voting for because the EU has been smeared even though I voted for Brexit. `It's very hard to cling to the facts during the political posturing of the most divisive general election since 1979, and the first to be subject to blatant foreign interference.