If I have a 20 year old plane(pretty new as planes go) and put in a new part, engine, propeller, tire, upholstry, injector, brake, etc.....That 18 years starts fresh on that part.
New engine? Whoever sold it to you has bought a liability tail for another 18 years.
And what of the total liability picture? Well, I'll tell you: it's been transferred to the owners, since going after the manufacturers is now harder (but, of course, not impossible). Liability premiums for owners are much higher, and many companies will no longer write the coverage.
Since I write those checks, I think the reality is painfully clear to me.