They can do what they want. Throw 15 years of iconic weapon designs and silhouettes away and start afresh, but then don't wonder why the game doesn't retain that same level of fanbase and passion.
How many people are going to make foam versions or costly weapons of the new green tube generic rocket launcher? Will anyone look at it and go, that's from Halo?
Karl raised a good point. There is advancements to old designs, those Reach concept art designs are good. They evoke the AR while looking new. They even have a lore explanation by saying these were pre-CE or experimental Reach designs. Think of how the Mark V armor went to Mark VI. But a radical redesign serves no purpose. Why is the new pistol black instead of silver? Because it looks like a gritty and common glock etc.
They're bringing the SPNKr back only after the fact people complained. Did no one think hey, wtf why are we even changing the rocket launcher design at all.