The difference between 2 years and 5 is quite significant for server runtime above water. This argument of diminishing returns would hold better for placing it 1 year rather than 2, 9 months rather than 12, etc.
If the concern is that you'd see significant failure after 5 (which is likely) which could undercut your future plans, I could understand cutting the plan short.
From my anecdotal experience working in data centers early in my career, you really don't really know if your failure rate for a specific type of drive/server is atypical for quite some time. This information is critical for many businesses, as choosing which drive or which architecture model can have profound cost implications.
It's quite possible that failures at the 5 year mark wont maintain a proportionate ratio with failures at the 2 year mark when compared to control. Maybe drive "A" tends to have an unacceptably high failure rate only after "X" hours of up-time.
If not much has happened after 2 years we're good to go. We need to get into the future as fast as possible.