The foam surrounds can be replaced. It isn't difficult and you can buy kits for a lot of different sizes including the glue.

One thing to watch is to ensure the cone is still centred but anybody who is patient can fix a bad foam surround.

I've done it! A relative of mine had two Gallien-Krueger 250ML guitar amplifiers, which are an interesting bit of kit, but the 5" speakers had foam surrounds that rotted out. I ordered and installed generic replacements from Parts Express. For my part, I was just curious about how the amps would sound.

As for the Bose, they were nothing special as speakers, and I already had a stereo that I was happy with, so I disposed of them.

Yeah, foam surrounds are the cheapo stuff, neoprene lasts.

They should use whatever bike innertubes are made out of. I've used innertubes as bungees outdoors, and they're quite durable.