Originally Posted by
dilanesp
But taking a seat in E+ makes that section more crowded and devalues the experience of the paying customers.
That's a good reason not to allow free upgrades to E+, but what the airline misses out on are the customers who would become loyal (read: eventually spend more on UA) if only they were allowed to "sample" a benefit.
Upgrades into F also devalue the experience of paid F but US airlines have made a different business decision for that cabin. Foreign airlines generally don't have free upgrades into F either.