It's basically the only shotty you use unless you're mini-sentry harassing with stock or widowmaker. I can see why they want to reduce its effectiveness to bring it in line with other weapons.
As an engie main who loves the Rescue Ranger to death, I kinda have to agree with this. Although I'll be sad to see this nerf, as I loved using the RR to heal buildings while low on or out of metal, in reality the nerf is deserved. Having a limited metal supply on the engineer is meant to be one of the engie's weaknesses; the RR completely negates that, as well as many other engie weaknesses.
Some of the other changes, however, seem a bit strange or have strange reasoning behind them: "This is one of our least-used weapons." This is the least-used Spy knife" "This weapon is one of the lower owner-equipped items in the game." "The second least-used item in the game." Some of the changes with this reasoning seem a bit whack, such as the Panic Attack's complete redesign. Yeah, some weapons are used less, but that's not always indicative of the need for a redesign. The Panic Attack didn't need a complete redesign. I thought the idea was neat, especially on engie, who uses the shotty as his primary instead of as a backup. Perhaps on Soldier and Pyro, who both use the shotty as a backup, it's not as used because the shotty needs to be there as a backup, but on engie I thought it was very viable as a surprise attack, in-your-face ambush weapon. As Uncle Dane once said in a video, Valve probably bases most of their rebalances on numbers on a screen instead of how it actually plays in-game. But I guess it's a good thing they're reaching out to the community before they release these rebalances, so we can say what we think of them.