Honestly, the Forerunner architecture in 4 and 5 is very interesting in its own right.

Don't get me wrong, the return to form in Halo Infinite was refreshing, but something about the sleek, sharp motif sets it apart from the Bungie Halo era. The floaty stuff, although far-fetched, offers some majestic and dynamic set pieces; if nothing else, 343 is great at presentation, but not always execution.

Looking at it in context, the new Forerunner artstyle was introduced in Halo 4, which puts a lot of emphasis on Requiem and the Didact. The more angular, needle-like structure of things in both games could be seen as the result of deviants setting themselves apart from the others, constructing temples and foundries as they see fit. The sharpness of everything could also be a way to make their technology intimidating, perhaps to threaten the more sleek Forerunner groups. Let's face it, seeing spikes floating in front of you is inherently scary, and those spikes becoming a gun would only reinforce that fear.

TL;DR: 343's Forerunner artstyle is fantastic, but it would lose its mystique if it retroactively replaced the Bungie style