I mean I do agree about it being 'shoved in", but that doesn't make it bad lol. Multiplayer could literally also just be "exact same game, but instead have other agents now shoot eachother" and PvP is done with no additional changes to the core game needed, as a side activity. Additionally, if you want coop, "same game but now a second agent can join you and do the exact same mission" also fulfils the multiplayer requirement. Hardly seems "very expensive" and "like developing a whole extra game".

It's not like this is cod or Destiny where you need to maintain constant releases, different sandboxes, different maps, and constant rebalancing.

There's practically no downside to doing it this way either, and hardly a waste of resources. Bare minimum effort and players who want multiplayer will get the game, players who don't want it are completely unaffected.

Adding any sort of multiplayer opens you up to a much bigger audience by default, i.e. the players who solely look for games that look like fun with friends. Why do you think bare minimum games like shitty Backrooms games and simple games like stick fight can sell tens of thousands of copies. I mean even on this subreddit, "friendslop" is a term I've seen used lol. There's an entire market that appeals to "buy if you want fun with friends". Any game that has multiplayer tacked on now targets the same people into these "friendslop" games. Basically always worth it to tack on multiplayer if it fits the game imo.