My favorite example is here in the Phoenix metro area. We had DSL and cable internet pretty early compared to the rest of the country (mid/late 90s), but then things stagnated.

Then in 2014, Google Fiber announced they were expanding here and, all of a sudden, the local telco and cable companies (Centurylink and Cox) started rolling out fiber all over the place -- like pretty much overnight. Then Google backed off, and the incumbents slowed their roll too.

It was a on-the-nose reminder that these companies can move fast when they think a real competitor might show up.