Yeah it seems so to me your right. Among the detractors of the idea on the thread I sense of a sort of pessimism, and cynicism about Trek's prospects and that's truly disconcerting.
I don't think it's pessimism or cynisism, it's realism! If the writers cannot come up with good and interesting stories a new galaxy won't solve that problem and if they are able to come up with good stories why would they need a new galaxy? All that would change would be terminology like "slipstream/transwarp 27" instead of "warp 9.8" and "andromeda sector 28" instead of "sector whatever".
Why not Voyager had the Delta Quadrant, DS9-had the Gamma, TNG-Alpha/Beta.
But the reason Voyager had the delta quadrant was because the ship was supposed to be stuck somewhere far away from home and DS9 had the gamma quadrant because the wormhole to another part of the galaxy was supposed to allow them exploring and meeting new species from a stationary setting.
If a ship just flies to the andromeda galaxy to explore it doesn't have to, if the premise is exploring it can be done in our galaxy.
Another galaxy is just the next step forward. If people can't see that they probably don't believe Star Trek can do it and has exhausted itself.
It's an unnecessary step forward when Star Trek hasn't even scratched the surface of our own galaxy. Voyager was in the delta quadrant for example but that doesn't mean it's explored, they just visited a few planets in tiny pockets of the quadrant.
I think you don't really grasp how big our galaxy actually is. It has between 100 and 400 billion stars, let's assume just 100 billion. Life seems to exist pretty much everywhere in Star Trek but let's be conservative (in sci-fi terms) and assume that 0,1% of those stars has a planet where intelligent life evolved and out of those civilizations 5% are warp capable while 95% are not, that still leaves us with 5 million warp capable space faring species.
If starfleet manages 10 first contacts per day and distance wasn't a problem it would still take them 500,000 days or 1,370 years just to say hallo to everyone with a warp drive.
Of course Starfleet is also interested in the 95 million pre-warp civiliziations and uninhabited star systems are needed to establish colonies or outposts. And they haven't even studied nebulae or weird phenomena yet.
The galaxy is unbelievably huge, there is absolutely no need to go anywhere else. Just think about TNG and how often they ran into unknown or weird stuff in
known space, it's very easy to miss things in space, it's that big.