And some times they keep the same designs for decades, with minor upgrades here and there. Take the M16 rifle, which has kept the same basic visual profile and general function since the 60s:

Or the M1 Abrams, which has been the U.S. MBT since the 80s, receiving upgrades on occasion but still looking roughly the same. Or the B-52 bomber, which has been in service for 60 years. Or the M2 .50-caliber machine gun, which has remained relatively unchanged for nearly a century.

The SPNKr had been in use with the UNSC since before the war with the Covenant, so that's at least 30 years of service. It was apparently a good enough design to keep around that whole time, and even after the UNSC went nuts with redesigning everything and making new ships and arms, they kept the rocket launcher the same. Until now. So there is no real justification for the new rocket launcher. It doesn't make sense from the perspective of the fiction to replace an obviously reliable design with a model that has a radical new design (single reusable tube, rockets in a two-round detachable box magazine). It wasn't for gameplay purposes as we've seen that it's functionally the same as the SPNKr and the fact that smart-scope isn't always used (see the sniper rifle). No, the only reason we have this new rocket launcher is because someone at 343i's art department wanted the rocket launcher to look different and didn't care what anybody else thought of their decision, and was given the discretion to act on his wishes.