It’s incredibly expensive to start an isp yes

If there were no issues w/ permitting, it's really not that expensive.

This article mentions some price estimates, which are $60k/mile (for buried cable) or $1-1.25k/house (excluding connection costs to the residences) (this one gives lower estimates). My city has 120 miles of roads and ~10k households, so the cost for my entire city is something like $7-13M, without hookups. Let's also assume it costs $1k/household to connect from the street (a figure I've seen quoted), so that brings the total cost to $17-23M for my city. If we assume the average household signs up for $50/month service, the payoff period is <3 years and average profit is 15%, we end up with 18-25 years to become profitable. I'm guessing this estimate is high since that profit margin probably includes startup costs, but 18-25 years to turn an initial investment into a 15%/year profit generator isn't bad, especially for something with inelastic demand like internet service (I'm guessing it's closer to 10-15 years).

I don't think internet should be a nationalized service, or even a state-provided service, though I do think there's merit to cities owning the physical infrastructure in their cities and allowing companies compete for customers on that network, mostly because them owning infrastructure presents a conflict of interest for direct competitors wanting to run their own lines. Larger government entities can and probably should own the backbone lines, but governments shouldn't directly provide the service because it's generally bad if you only have one option for a given service.

MS got to where it was because they buy up competition

Not really, they got there by directly competing w/ IBM to build marketshare, and then contracting with PC companies to pay Microsoft a fee whether they included Windows or not. The only acquisition Microsoft made around that initial rise was for a company that made a presentation program that eventually became PowerPoint, and that's it until the Dept of Justice filed it's first official anticompetitive impact statement. Microsoft dominated the web browser market in the mid-90s because they were one of the first to market, but they abused this position by locking out competition from internal APIs that enabled better performance (2000 antitrust case).

Almost all of their monopolist behavior was with products they built themselves and locked people into, not products they bought to stop competition, that nonsense didn't start until fairly recently (i.e. Github and Activision/Blizzard). For most of Microsoft's time, their monopolist behavior was simply abuse of their market position to either force companies to pay Microsoft even if they didn't ship Windows (thus pushing them to ship only Windows) or locking out direct competition for their products on their platforms.