There are multiple arguments being made here. Yes, if you went out and signed Greg Maddux to a 20 year contract right now, technically, he would be part of the "future".
The most efficient use of a free agent signing is to sign them when you have a point of need on the win curve. The way teams are built now is that they sort through their internal options to determine where they have holes, or need depth. It now takes 85-88 wins to make the playoffs. If you are a .500 team, obtaining a 4-5 WAR player or two puts you over the win curve. That is the point where those wins are most valuable. If you're a 50 win team, adding 5 wins doesn't change your team's ability to make the post-season.
So, what you are doing is adding 4-5 wins to a 50 win team. (Some here would argue they're a 41 win team, or less.) My highly optimistic view is that the Sox won't really be able to use those extra 5 wins to put them up over the win curve until 2027 at the earliest. Others here would argue that critical year might come in 2028 or maybe not even until 2030, either when Kim's skills are diminishing, or he's even gone.
Pretend the Sox can contend in 2027. You're paying for 2 full years of 5 WAR production until his contribution puts you over the win curve, and probably for diminished production for a couple of years afterwards. The most efficient use is to hit the youngest and most productive seasons in the first years of his contract. Are you saying there won't be shortstops on the market in 2028? You seem like a fairly positive guy. The Sox will draft #10 this year, top 5 next year, and maybe #10-15 in 2027. Will there be no good shortstops available in the first round in those 3 drafts? Will we not get better at developing prospects and have a package to trade for a shortstop by then?