I don't typically see computers getting replaced because they are "slow" but because of factors more related to support like apple dropping support for older models just because of reasons. Or companies doing routine system replacements because their fleet of computers are getting banged up and damaged and since they are XYZ years since their release and are no longer in production their costs and complexity with repair, maintenance time, etc... Are no longer worthwhile. Not to mention, the more insecure members of our species feel their self worth is dependent on how fancy and new their things are.

If you want to point fingers at why performance for operating systems and programs has declined over the years I would say it's mostly in part due to security functions.