A cold air intake is more for just that, cold air. The longer you run a car (let's say a track setting), the more heat the engine will generate, and a greater need for cold air will arise. I'd argue that in some cases, a cold air intake would improve an engines lifespan.
This is a good point, and is why the idea of "[HP] gains from a CAI upgrade" in a street car is kind of laughable.
To the extent that an aftermarket air intake makes any difference for a street car, unless you're a maniac running it at high RPMs for sustained periods of time it's not because it's adding
colder air, but allowing
more air to get to the engine.
Which in turn, is because they also come with a different air box, replacing the stock air filter with a "high flow" one, or even no filter at all.
That's what makes it "sound better" and potentially to "add a few HP". Not the "air reaching engine is now colder because it comes from farther away!" angle.
And yes, less filtering of the air to your engine CAN reduce its lifetime, by some amount and over some duration of time that may or may not matter to you.
Will it allow some particles to enter your engine that the finer, more restrictive stock filter would have caught? Of course it will. That's literally the tradeoff you've chosen to make.
Will those larger particles now entering your engine, do damage? Yes, but only over time. It's not like they're rocks that jam things forever. The particles will burn up and go out the tailpipe, the smaller the faster. But until they do, they're going to be like having extra grit in the cylinder chamber as the engine works, which is where the damage comes from.
As for whether adding a CAI needs the ECU to be retuned, the answer is generally "there is no point to doing this" because unless you've also modified your exhaust, letting more air come IN to the engine has an obvious ceiling as to what it can do unless the "post-processed air" can go OUT of the engine at the same rate. And the exhaust is much more restrictive in flow than the air box is.
Basically, "just throwing in an aftermarket CAI and leaving everything else stock" bumps your airflow up towards the max permitted by the stock exhaust, while allowing larger airborne particles to get into your engine.
Is that worth it? That's up to you!
Modify the exhaust to let more air flow at
both ends, now, that gives the engine more "air" to work with in the "air + fuel + spark" combination that makes all the magic happen for a combustion engine. Along with playing with the fuel (octane) and spark timing (and compression for a turbo), THOSE are the things that require "tuning the ECU" to make use of.
