eventually the foam rubber surrounds do wear out, either just losing flexibility or actually tearing. you will be able to buy replacements when that happens; proac just uses scanspeak drivers - although they're branded 'proac' and might be a little different (I'm sure they'll claim they're really different).
one thing to consider is whether the speakers are exposed to full sun; that can speed up the deterioration of the foam surrounds. if one gets more sun than the other, switch the speakers every once in a while to even out the exposure.
otherwise, it's going to be a function of how much you play them and how loud. there's no reason you shouldn't be able to have these speakers for decades if you replace the drivers when needed. most of what you're paying for with speakers is the cabinets, not the drivers.
p.s. when you do replace the drivers, have it done by a qualified techlll
one thing to consider is whether the speakers are exposed to full sun; that can speed up the deterioration of the foam surrounds. if one gets more sun than the other, switch the speakers every once in a while to even out the exposure.
otherwise, it's going to be a function of how much you play them and how loud. there's no reason you shouldn't be able to have these speakers for decades if you replace the drivers when needed. most of what you're paying for with speakers is the cabinets, not the drivers.
p.s. when you do replace the drivers, have it done by a qualified techlll