Who says it would even be Starfleet who restored the ship? It could've been outsourced. I think in-universe, someone would want to restore the second most famous Enterprise if they had the means to. I'm from Massachusetts, and I visit Plymouth from time-to-time, where they have the Mayflower II. It was built in 1957 and is a replica of the original Mayflower that brought the Pilgrims over. Do they need it? No. Did someone want it and was able to make it happen? Yes. Need and want are two different things. My god-sister's husband is a car guy. A huge car guy. He loves restoring classic cars. It's a hobby. People like this probably still exist in 2401, even if it's not with cars.