The problem of employers not wanting to hire older people may not be so much of an issue - they may not have much choice.
People's fitness will be. However, there are a good number people who manage to have rather long retirements who are fit, so it may contribute some, and we may see changes in how we manage employee tasks to account for fitness.
All this being said, part of the issue is that while we have increased the lifespan, it hasn't all been an increase of healthy productive years. We've added on a good number of years at the end which are often rather miserable and require extra care.
I wonder if we may see a return to certain older ways of doing things - older people moving to very small flats or flat sharing, or moving in with kids, for example, while taking on less demanding or part time jobs. The idea that working forty years entitles us to another 25 or 30 in our home having fun and then being cared for may just not be sustainable, unless you want to be putting away a third or more of your income during those working years.