Delpheas said:

For one, it’s a major way the film apes ANH, but on a more ridiculous scale.
Second, it has no emotional impact. Just adding in the deleted scene of Leia’s messenger being sent there doesn’t do enough to give the audience an emotional anchor. And again, it has no bearing on the later films. The only reference to the NR destruction in TLJ is the crawl and a Hux line toward the beginning. With the NR still intact throughout 8 & 9, the lack of response from allies can be recontexualised by either editing or the viewer as the NR continuing to not help. And the arrival of the fleet over Exogol in RoS could be contextualized as the NR finally sending aid.

And finally, I think the destruction of the NR was a very shallow way that the writers were signaling that they weren’t going to be beholden to Legends material, since the NR plays a not insignificant role in Legends. They then of course still borrowed the concepts of Jacen Solo, a hidden fleet, force clones, and Palpatine’s return… I just really feel like it’s useless and goes nowhere.

Personally I think removing the Super Weapon concept at all, if possible, is ideal. Starkiller would just be a FO base that they assault to rescue Rey and because they might have a chance at killing Ren and Hux. Idk…

Alright, I’ve been going over this the last couple days and looking at not only this film, but the trilogy as a whole with and without this scene. I think I’m going to keep it in, and here’s why.

Based on my plan for Episode 9, I want the galaxy to end in a state of total reset. No Jedi, no Sith, a gap and void for something new to rise. With the destruction of the NR, I think that sets up a more broken galaxy that is ready to rest and recover. It adds more desperation to the Jedi v Sith conflict and when it ends, there’s hope for something new, something to break the cycle of these force wars.

To address your concerns:

Maybe if you only look at the movies Starkiller firing seems to try and unnecessarily one-up ANH, but in the context of Legends stories as a whole it doesn’t even need to be compared. It makes sense as a natural evolution of the Sith to move up to something of a grander scale.
True, they emotional impact of a whole system being destroyed isn’t very present, but I wouldn’t say the movie needed that to be felt by our protagonists either. The main conflict is with Rey, Kylo, and Han reflecting the light v dark conflict, and this destruction paralleled with the death of Han goes along the thematic lines of the dark securing a victory.
To be honest, I doubt the writers even thought of it that deeply. I don’t think they were smart enough try and intentionally make a statement about the NR of Legends through Starkiller base. Abrams didn’t even know who Darth Plageuis was after TFA so I doubt he knew much about post-RotJ novels. I don’t see it as necessarily disrespectful to Legends, more of just an alternate reality.