I think that's part of the progression, though, you sort of outgrow the ghetto pipe weapons and move into proper weapons, and that second tier that the combat rifle sits in is good enough for end game with upgrades.

Otherwise a player might feel good enough sitting on pipe weapons all game and subtly feel less progression and satisfaction from the game due to lack of visual novelty and progression in weapons.

However, they probably could've added more upgrades to keep using the pipe weapons, but have them unlock late or through specific quests you'd have to seek out, and make them very expensive material-wise to disincentivize sticking with the same weapon all game, but still allow it.