Especially for game designers or software engineers, I was wondering how significant of an increase in difficulty/frustration/workload it would be for multiplayer to be an afterthought of a singleplayer game, as opposed to putting in all of the work for the game to be "multiplayer ready" from the start.
This question is especially important when the question of "Will there be multiplayer?" is up in the air until after the game is developed.
I only ask because coding with multiplayer in mind from the start, I assume, is a hefty chunk of extra difficulty that one could otherwise skip. Of course adding in such a engine-changing feature, I assume, would be quite frustrating given a game already close to being finished. Both strategies seem burdensome, but if the extra workload of implementing a big feature after a game is already completely is extraordinary compared to the heavy workload of engineering it from the start, then obviously one would nearly always want to prepare for multiplayer.
I am also curious on a personal level as I have seen a few indie projects declare they will never add multiplayer, and then months later after the game has over a year in development, change their mind and say they will be adding it as a high priority feature due to the large demand for what is probably the most common core feature of any game.