Thoughts on extending speaker life.

An occasional application of Armor All is beneficial for surrounds.

I would never apply anything to my speaker surrounds. It can do more damage than good. As long as you keep your speakers from exposure to any of the known things that can cause degradation of the surrounds, the surrounds should last a lifetime. The things to avoid are UV (from sunlight and fluorescent lighting), Ozone (laser printers are big culprits), VOCs (from paint, household cleaners, furniture polish, metal cleaners and of course cigarette smoke) and excessive heat and humidity.

I purchased my KEF Calinda speakers in 1979 and they are still going strong in my third system. The surrounds are rubber and still look like the day I purchased them after 40 years. My KEF 104/2 speakers were purchased in 1991. They are the biwire version and have foam surrounds and foam donuts on the woofers. After 28 years they still look like new and show no signs of degradation.

Don’t forget about the Most important part of the speakers,
the Heart ,the Crossover , Parts Quality is average at best 
on 90% of all crossovers, upgrade withsame values just much 
higher quality  parts ,including , wiring, and getrid of the cheap gold plated brass that most speakers connectors have as well as your electronics rca and Loudspeaker terminals ,that alone a very noticable sonic improvement.

AR 18S surrounds crumbled after maybe 10 years. Mission 761i will be 25 (years) in a month or so. surrounds look like new. Third pair, 14 years, looks like new, too. Fourth is still too young to tell. None of them has ever been treated or paid any attention to.

Could it be that something about materials used has improved over the last 20-30 years?

I would not apply anything on surrounds. There is more chance I would damage them, poke the hole or something else disastrous, than that I would extend their lifespan by a few more weeks. I do not think it is worth it.

You should be more concerned about components, as the odds are that at least one of them will go first. Silver lining in that is sound quality in general for the dollar has gotten way better in past 5 years imo across the range of gear. Spend your sleepless nights dreaming of a DAC crash and upgrade.

If an older speaker died, count your blessings and move up!