It doesn't cut your trade income in half, it doesn't even shorten your trade route. For the Nova to have any value as eco hamper you have to invest into at least three or four of them, the same money would grant you a large fleet capable of actually taking away planets from the enemy.
Ever since Rebellion beta started people keep getting spasms over "how powerful" titans/superweapons are. They are not. A Titan is taken out by medium to large sized midgame technology fleet. I personally don't understand the "but it feeds titan" whining since your fleet grows over the course of game as well, so unless you are economically broken the issue is not as big as people blow it up to be. Not to mention your own capships and titan level by exploding titans as well, even faster if the destroyed titan is higher level. In a defensive position your losses are bound to be less expensive than the opponents, too, unless you mess up big time.
Novas and other superweapons are unchanged compared to diplomacy, if anything their effect has become weaker compared to then since everything else seems to have been buffed. The main value of supers seems to be the suprise effect, distracting an enemy with fortifying their worlds for a few moments so you can prepare an attack.
People need to overcome that "OMG giant ship/warhead incoming what do I do OMG" fear and actually take a look at the things, maybe they'll start becoming used to devising a strategy against them.