Interview 1
In a March 2018 ESO Live episode, we learn that the Creative Director of ESO, Rich Lambert, considers Veteran Maelstrom Arena to be one of his favorite pieces of content. The two hosts of ESO Live both mention that, at the time, Lambert had run VMA over 600 times, and Lambert confirms counting his runs. He then goes on to say, "...[Veteran Maelstrom Arena] is definitely one of my favorite pieces of content."
Source (VMA talk begins at 11:30):
Interview 2
Lambert also mentions in a recent Twitch interview from last week that he runs VMA "a ton," re-confirming his position from two years ago that he engages with the content at a high rate (likely for enjoyment, as the reward pool must be severely diluted after so many runs). In the Twitch interview, Lambert also avoids giving a clear, justifiable answer as to why the existing VMA weapons are not being updated.
Source:
Likely Conclusion
The absence of a justifiable reason for not updating existing VMA weapons, along with Lambert's voiced affinity for VMA shines light on the likely reason existing VMA weapons were not updated -- Lambert just wants to give himself further/renewed incentive to keep playing "one of [his] favorite pieces of content" that he plays "a ton." Go ahead and also toss in further evidence from that discord back and forth that he had with a player where he claims he is in the same boat as everyone else -- implying that he has to run the content again if he wants the weapons (something he already does, clearly enjoys, and chooses to engage with frequently -- unlike many players).
Lambert's decision not to update existing VMA weapons appears to be made based on his self-interests -- not the interests of the community of ESO players (given the feedback that we have seen on the forum regarding this topic).