I think you're right about trendiness and hubris associated with UI redesigns, but I think it might be a mistake to imagine this is some sort of one-time sea-change.
I think it's more of a constant churn; those who would "modernize" or "update" all sorts of designs - from software UI to software architecture to real architecture to interior decorating to graphic design and brand identity work - always have (arguably need to have) an unjustified level of self-confidence, a high opinion of their ability relative to the task they've got before them. They're always blind to the tweaking and tuning that was required to produce the thing they've set out to replace.
Most of the time, if they had an accurate idea of how monumental a task they were facing, they'd look for something else to do.
I think it's more of a constant churn; those who would "modernize" or "update" all sorts of designs - from software UI to software architecture to real architecture to interior decorating to graphic design and brand identity work - always have (arguably need to have) an unjustified level of self-confidence, a high opinion of their ability relative to the task they've got before them. They're always blind to the tweaking and tuning that was required to produce the thing they've set out to replace.
Most of the time, if they had an accurate idea of how monumental a task they were facing, they'd look for something else to do.