It will never sag........
That frame is probably more durable today than it was when it came off the factory floor.
When these trucks are stretched like this, the frames aren't just cut, a 3-4ft new section added back in, and then welded together. They are Cut, the 3-4ft section welded in, and then a minimum of 2 "X" braced frames added connecting the extended portions together. Depending on the Shop that did the work, the new welding is probably done better that the computer ran welding machines the factory uses.
Take Shipshe Welding there in Shipsewana Indiana for instance. They do Frame stretches like this all the time, and could have very well done this truck above. Those Amish Guys they have working for them do such a good job that it would probably take TNT to break the welds off.
I've seen, worked around, and know dozens of Drivers who's had their 1 ton trucks stretched to add sleepers on them who have never had a sagging frame develop after doing so. Only 1 I've ever seen that had problems was these 2 girls that ran together over at RAD Transport delivering RV Trailers. To save money, they had theirs done by some backwoods Welding Shop in Kentucky that screwed it all up. The Frame would bounce on them going down the road and the sleeper kept breaking away from the cab. I believe they finally had Shipshe correct for them after just a month of running like that.
These trucks are out there everywhere.....keep your eye out...you'll see another one sooner or later.