can a sub ever blow off a good signal?

If you overpower it then yes. Your sub should last if you give it a clean signal within it's mechanical and thermal limits.

The soft parts will eventually break down. Spiders and the surround mostly.

Yep the surround is usually the first to go. I have several subs that work fine but the surround have disintegrated completely.

That being said, the soft-parts will usually last QUITE some time. If you keep the sub with-in it's mechanical and thermal limits, and keep a clean signal to it, there is absolutely no reason it shouldn't last several years.

I've had my 15 inch SI Mag since 2006. Still looks brand new and sounds the same. So that's 7 years, no problems

overtime the spiders lose strength. it'll take years, depends on its usage, but as long as you keep the subwoofer within its rms range it'll be fine, most subs can take way over its rms, but not for very long portions of time, for example, i can burp 2.2k on my 08 fi ssd which is rated at 800 rms. but thats a clean signal and the subwoofer gets warm to the touch after a few times.

people often overlook environmental factors. Sunlight, moisture(humidity) and heat from the sun.

Where do you have the gains set on your APSM? I wouldn't exactly consider that clean power...

But like it's been said, most subs can take quite a bit more power than they're rated for if the power is totally clean. However, even with a clean signal, you're probably not going to be able to throw 2500w daily at a 600w driver. It's all about the subwoofer's mechanical and thermal limits - if the sub is pushed too hard, it can start leaving it's gap or bottoming out, potentially damaging the subwoofer. After a while, all of that power causes heat, which will eventually cause deterioration of some soft parts after long periods of time.