Only the best organizations in baseball are capable of doing both, I'd agree. The Rays graduated 4 prospects off BA's top 100 last year and still managed to sneak 5 more on their 100 this season. The Dodgers, Braves, Astros, Yankees etc are other examples of organizations that consistently win in the majors but keep their farm systems strong. I'm not looking for the Sox to be on that level, but they're likely to be the only team in baseball that won't place a single player on any of the top 100 lists that come out this winter. Very few prospects in the system took meaningful steps forward last season, which is how you end up in a situation where the Sox are forced to deal from their big league club at the deadline last year and likely would have to do so again if they want to make a meaningful addition via trade.
It's not unreasonable at all to expect the team to manage being a contender with a top ~25 system, which would just give them enough ammunition to make trades without having to deal away players that are essential to winning over the next few seasons.