Not all the investments of the dotcom bubble were malinvestments. A part of it has been re-used, but another big part was wasted. There was no need for all the crazyness and all the waste to create the infrastructure we have now.
But this applies to everything. Even in non-boom times, a large part of investments is wasted. I've come to see it as a kind of decentralized, randomized discovery process. A boom/bubble is just a speedup of the normal rate at which this happens.
On the other side you have the structured, committee-based economy, which tries to produce growth at a fixed rate by guessing what things are real and useful and which are bubbles and shouldn't have resources wasted on them. The problem is, that it is hard or impossible to know those things in advance. Good ideas might be shot because "they're just a bubble" and bad ideas might be preserved because they've worked in the past and the world has changed since.