The one entity that is not going to invest in new plant is an entity that has an existing plant.
For instance if there is a phone company with a copper plant that can charge $60 a month for DSL it is obviously unaffordable for them to upgrade to fiber so they can charge $70 a month.
Some other entity might find the new plant is worth investing in but it still fights against the old plant. For instance the phone company is making crazy profits on $60 a month DSL and can probably still make profits on $40 a month DSL. This puts a downward pressure on pricing for a fiber service and also means the fiber plant is going to bypass many possible subscribers because there are plenty of people who will put up with a 100x drop in performance to save a few dollars.
This book
https://www.commentary.org/articles/leslie-lenkowsky/the-zer...
works an interesting case study that was fundamental to the Democratic party becoming advocates for free trade. In particular, the steel industry in the US in the 1970s still depended on open hearth furnaces that were obsolete in every way to the basic oxygen furnaces used in Germany and Japan.
From a marxist perspective, capital is the "master class", maybe even the capital itself (as opposed to the capitalist), embodied in those furnaces. Instead of replacing the furnaces, the US steel industry pursued protectionism which was harmful to the competitiveness of other sectors of the economy: inferior and expensive steel is an ingredient for inferior and expensive cars, buildings, etc.
To analogize, however, the problem is that the copper phone network exists. So long as it exists there are strong incentives to keep using it. It might not be a problem of encouraging investment in fiber as much as forcing disinvestment in copper.