You could think of a system by which multiple ISPs could sell service over the same city-owned infrastructure, but there's no real benefit to that. (And I say this as someone living in a country where that essentially happens.)
I mean I suppose you could ask the people that voted for it.
>The solution should be raising competitiveness of the market.
Networks like internet providership are notoriously hard to get competitive markets in. Even removing the regulatory barriers to entry there are still natural barriers to entry (high start up cost) and huge returns to scale. The natural state of this market is monopoly or oligopoly.
But even beyond this, this is raising the competitiveness of the market. Internet has become so integrated into the fabric of modern life that many feel compelled to purchase it from the one or maybe two providers in their area. That lowers competitiveness. Demonstrating that municipalities have ways of 'opting out' should force Comcast and other providers to compete with the hypothetical 'public ISP' that citizens can always vote in.
I would personally believe that if there are issues with quality of service or price or whatnot, this governance style allows for a much better opportunity of addressing issues. Versus being run by a very large monopoly that honestly may not care that much about the ongoing needs of a 1200 population town (read: small revenue source) in the future.
A competitive environment would be better if possible, but failing that, I personally would take a municipal broadband system with a good degree of accountability built into the town governance over an oligopoly.