Why well-maintained Mersey ferries still needed replacement

Why replace the Mersey ferries if they still appeared to be in good condition?

They had been well maintained, but their service age was recalled as roughly forty to sixty years.

The ferries still appeared sound because they had been well maintained. Their age nevertheless supported replacing them. Alex Clarke first estimates forty or fifty years in service, then allows that they could be sixty years old while trying to remember when his father worked on them.