The trick still works (and is quite common), because investors apply that reasoning to the upstart, not the established business. If you go to a bunch of VCs and pitch them "Comcast is making a ton of money because they own a monopoly and steadily raise prices. Their margin is our opportunity. Give us $20B so we can build out a national 1G fiber network and take all their customers", their response will be "What's to stop Comcast from dropping their prices and increasing their bandwidth once you've spent all this money building your competing market?"

Knowing that you're the attacker and they're the defender and that most consumer markets favor the incumbent, the investor won't hand over their capital unless you can show that you've already started to take their market. And then once you have shown that you're taking a fat incumbent's market, they're happy to fork over tons because they know that every other investor's reasoning will be the same, and nobody will want to attack an incumbent that can just drop prices to fend off a challenger.

BTW, Google decided to buck the outside investors in exactly this example, funding GFiber internally. It played out basically exactly as the scenario above - the incumbents rolled out gigabit cable/fiber at competitive prices in precisely those markets GFiber was threatening, preventing them from making serious inroads. There were a bunch of other factors (like regulatory issues and the difficulty of scaling last-mile telecom), but after recent examples like that investors would rather find virgin territory rather than fight price wars in large existing markets.

(IMHO, this is one thing wrong with American capitalism today, as our system relies on cutthroat competition and aggressive risk-taking behavior by investors to get good deals to consumers.)