The Boss, if left stock, will retain more value than a standard Mustang, simply due to the low production numbers for the car. That rule applies to any car made in low production numbers. It has nothing to do with age, or ego. It is how the American auto industry has worked, since the beginning.
Now, if you are going to mod the car, then the Boss is a poor choice as you will wreck the value retention, as it is based on keeping the car stock.
No one said you are going to make money on the Boss. Not going to happen, but if resale is a concern, the stock Boss wins over a modified Mustang, in terms of retention.
Of course, if you are a collector, the Boss is a no-brainer. Very little risk in adding that car to a collection.