The rest of us believe that telecom is, was, and (for the foreseeable future) always will be a *natural* monopoly
Natural monopoly is a myth. A myth very convenient for and thus perpetuated by the government officials of various levels as it gives them undue power, but a myth nonetheless.
You can't have meaningful competition for building roads and sewers and power grids
Yes, you can. Tokyo has competing subway lines — why can't New York City? Your GPS is likely to show you several options for any route of appreciable lengths — why can't those different roads be privately-owned and compete?
For example, to leave New York you have many options (most of them requiring payment on top of the taxes) — why can't those bridges and tunnels be privately owned and compete with each other? Maybe, their new owners will consider high traffic a profit opportunity, rather than a burdensome nuisance — and seek to attract more drivers by innovation of both toll-collection and road-maintenance... I dunno, it works for supermarkets... Heck, some private (and disgustingly profit-driven) concern may even undertake building a new tunnel (or a bridge)...
it will always be vastly more efficient for a single entity to install and manage that physical data network, at least at the local level
Really? Why not? In the 20ie we had competing telephone companies — each running its own wires to buildings. Today Google is laying down its own fiber — to much rejoicing on this very site — and AT&T is planning its own alternative, despite your claims of it being "inefficient". Various markets have competing coax-cable providers already. The actual cable-laying is just a (small) part of providing Internet service... Though in theory a monopoly ought to be easier — and thus cheaper — to operate (in any market), in practice any benefit is quickly consumed by the inevitable arrogance of such providers and the concomitant drop of quality and rising end-user prices (any wins in the monopoly provider's costs are compensated for by their fattening up the profit-margins).
We should have made this transition decades ago, but for a variety of reasons didn't
Oh, it is not a "variety" of reasons — but a single one: our government followed that myth of "natural monopolies" and granted cable-TV providers monopoly rights in their respective markets. That law was rescinded in the mid-1990ies, but the damage was done...