Maybe you weren't around for the initial kick off of NL (5 years ago), but Messana explicitly said the purpose of NL "was to bring UO the biggest audience ever"...so you would be incorrect in your original thought/design of NL; it was absolutely (initially) meant to bring in additional subs.
We are in agreement though that I don't think that's what NL actually is (I don't think enough people know what UO is to even know about NL or want to sub). Will it be some sort of quasi test shard? Maybe. But the fact remains that Kyronix has referred to the "spaghetti code" multiple times and has inferred that they aren't interested in touching it...so there will be many things that I don't think will ever actually be sent to Prod. The quest system is something I'd doubt because there are currently at least 2-3 different quest types on Prod now that they would need to find and standardize within that old code. Is it possible, sure....is it likely, probably not.
I was not playing 5 years ago, but I was aware of it around that time (maybe a bit after, it was whenever it showed up in the newsletters, which I did follow). How do I put this delicately - having played a lot of MMOs since the late 90s, I know that lot of producers/devs say a lot of things, and I'll just say this much: I was there when UO was at its peak (and before and long, long after that peak). I didn't feel like NL by itself was going to be what got people back or got kids interested.
My guild went from UO to Star Wars Galaxies to Warcraft to Guild Wars, with a few detours here and there (EVE Online, etc.), to these days Final Fantasy XIV, and we still talk about UO, and what we wished had happened, and what would have kept us, etc. What our guild started out as in UO - looking out for each other in Fel before the Tram land rush, building a small town, working to make decent gold through some crafting, some hunting, some gathering, etc., eventually evolved into what you think of as a guild in modern MMOs today - lots of raiding and other large-scale activities. UO currently is not on the radar of people who expect the kind of raiding/activities you see in various MMOs these days. Part of what keeps UO off the radar of those people is what makes UO so wonderful, but part of it could use some work, and I think NL was meant to do part of that work. Whether we like it or not, things like a better quest system make a lot of sense, because for younger players (we'll say under the age of 35-40), they expect MMOs to provide them solid quest mechanics that get them deep into the game and keep them doing various things until they are comfortable with how the game works, with the UI, etc.
While NL sounded interesting, at times I thought it was aimed more at younger MMO players, and at fans of the stand-alone Ultima games who were looking for a RPG experience within UO (UO's reputation among fans of the stand-alone/Ultima prime games is a mixed bag) or even something like fans of Elder Scrolls Online. There was (to me) just something about it that - I feel like there is something missing from it - not necessarily a graphics or client upgrade, but something that needed to happen alongside it.
I had a sense that NL (by itself) was supposed to be something that allowed content to be delivered more quickly, and I don't mean shiny new items for the collectors, but actual gameplay content. That was my sense from Mesanna's various comments, even if it didn't always match what she was saying. From my experience in software and game development, and given that the team is small, my spidey-sense said that NL was just that - get more content in front of players faster, because the last 25+ years have seen many, many comments from designers and artists about how difficult it is to add content to UO. What that content looked like, whether it was quests or new dungeons or shard-wide events, I don't know.