First argument: "The whole system has to be recoded." - Please add a source or proof. You just state it. I state: No, that's not true. Not because I wanna mock you, or to make a hateful post.
I say that, because I think that a big part of the system will stay and be built upon.
- Less planet lag, large structure lag
- More stable gameplay
Are you technically experienced to know that? What is your profession? Everything over one year of work experience with unity should start talking now.
Please add profound proof to you suggesting thoughts. Or ask anyone that works with unity. And don't cite a wiki or some random forum walking stranger with 10 posts.
But lets have the noobie talk: (I have no experience about this too). Do you really think that unity can do a better job in rendering millions of blocks in a 500 diameter ship, or if a one-piece designed java engine, constructed only for this one particular job, might be better?
And stable gameplay? Do you really think unity will do a better job here? How do you know that? Do you know about the netcode of unity? I do not. We two have to assume so many things here. What if the netcode has to be written individually for Starmade anyway, because the data that gets exchanged is different from other multiplayer games? Same goes for the stability in general, not just netcode. A block game is different in its engine than typical 3d games. I think unity might be great for classical 3d engine games, but I have not seen a single big block based game that's finished for unity.
Important point: The dev has to pay extra for using unity. I hope you considered the financial aspect too, before suggesting this idea.
You know the thing is, you thought about the game. And you hope the best for the game. Be not worried. Starmade will be a game one day. The link Lancake posted is very usefull to give some insight. Anyway this "this or that engine or language" talk should really be done when there is profound knowledge involved from at least one of the participants. If I am the one with no knowledge, I have to be the one with questions to the experienced people. =)