We all know how to get Helioskrill in Halo 5 but it was ruined when they added different varients and you could get it from a pack. Should they add the Helioskrill into Infinite for those who got the legend achievement in mcc.No...why would anyone want to go through that again? Have YOU tried playing Halo 2 on Legendary?
It doesn't mean that we'll need to go through the whole thing again. All that you need is the achievement, not the progress.
OP: Yes, I think they should. Helioskrill was popular because of the effort but now lost that value due to knock-offs. However, those who got it and know the design will recognize those who made it through and who didn't. I know you did it because you have the original pattern.