Not sure if I like it. Production is kind of essential to Civ series. On the other hand, being able to build things instantly would be more realistic, by far. No more hundreds years to build, move a settler, grow a new city, etc.
In the eventuality it is a thing, then the map would become more strategic, but a problem could be german panzer blitz attacks -likes for every DOW. To counter this menace, you would have to build up defenses all around your territory, like walls, forts, etc. and pre-organize ripostes with pre-orders in case of DOW. (That would be a dramatically different game than Civ has been until now.) It would basically become a war/cold war simulation without the "pushing units" philosophy of Civ. Because units should be dematerialized to some degree, for example they would all act as planes do now with a radius from cities/your territory/defensive places.
Another option would be to keep production, but being able to collectivize it between several cities. That way, you may be able to produce x units/buildings in one turn, provided your cities are linked properly. The first move after a DOW would be then to cut off cities connections, or you would face up a continuous flow of units that could quickly become boring to eliminate. Of course roads should still cost maintenance, because otherwise it would be too easy to prevent cities disconnection. A blocade could be made by putting a military unit adjacent to a connecting river or on a connecting road. But a DOW could quickly become a disaster for your civ if you didn't plane well/protect your connections. Again, this would become kind of a cold war every game in every epoch. Could be interresting, especially if you link the protection of your connections by Civ6-walls-buildings-likes if your connections are naturally exposed (in the case of rivers for example) But the balance between impossibility to break connections so undergo an overwhelming flow of units if you're the declarant and ability to protect your own connections if you're declared might be impossible to find.
Unless it has not to be found : let's assume that during a war, your connections are broken automatically or very easily by your enemy : this could go the same for him, and production of units during war would slow down significantly, not by realism entirely, but by an effect of lens that would zoom in the conflict, regarless or years passing each turn like it is now. Note : the difference between connections being automatically cut during war and the relative easiness to cut them off is significant : an exploit could be to DOW your enemy and it would be crippled, but so would you. If it's automatic, then you want to prepare well upstream a war for it to last the littlest number of time possible, because it would cripple even your wonders building ability at this point. If it's not automatic, then you would have to prepare well and protect your connections, with units along them (a lot of units would be less costy than in previous Civs) but again, even without buildings dedicated to your connections, it could be a nightmare to balance and wars of invasion could become impossible.
Third, you could, with this second option, turn the game into a tower defense in times or war/cold war, with a continous fow of units going where they are told, fighting automatically, winning or being defeated, but most probably defeated at some point, until you finally reach a city center. The difference would be in the collectivization of resources through cities connections, that you could still try to disrupt with special objectives/aims other than core cities for your troops. On contrary, instead of totally scrap production from Civ, it would emphasis it, and the player would be more careful for his city placing, or at least I know I would be, because in other Civs I don't always take care about the potential production a new city may have, early or even later.
