Thoughts on extending speaker life.

I agree with Elizabeth. Though I can't comment on the advisability of applying silicone to the surrounds, it is a bad idea to apply petroleum-based products unless you can ascertain the exact composition of the surround - not all "rubber" surrounds are created equal. 

Some like to rotate their woofers occasionally, to counteract potential suspension sag. From everything I've read and encountered, it takes many years for a good suspension to suffer that phenomenon. 

One of the best things you can do to prolong woofer life is to keep the grills installed, at least when not listening. Many speakers are voiced for use with the grills intact anyhow. In fact, 4 of my 6 pairs are voiced that way and sound better with the grills in place.

You have great speakers, forget about  it.

Listen to music, life's too short to be concerned. 

Perhaps in 20years, when the drivers show their age, you can then seek out a complete set of NOS drivers.

Oh but wait, those too, have been sitting around...

I have a pair of KEF Q10s bought new in '94 or something...I originally used them as near field monitors in my studio...these things are maybe 25 years old and still sound fabulous with the rubber surrounds pliable and seemingly fine, showing no signs of age other than the minor scratches and dents here and there, and now I stick them in the windows to my deck for outdoor speakers when needed (driven by a designated mosfet power amp). Great speakers.