This remains my key concern. It strikes me as absurdly difficult to get to the top if a team is not willing to supplement at all in the serious free agent market. I think teams can (and is likely most sustainable) build the framework of a serious contender through draft, develop, and extend, but its hard to be a top 3-4 team without any major free agent on the roster.The filling in the blanks part is tough, though. Sometimes you sign a player with warts to a 5 year deal and they become JD Martinez and anchor a WS lineup. Other times they become Yoshida or Story. I'm not sure you can completely ignore the second tier FA market and win, but it's certainly fraught with uncertainty.
The Red Sox teams of the past that have been great had these players (e.g., Manny in 2004, Manny and JD Drew in 2007, a number of mid tier guys in 2013, Price in 2018). Its likely also the reason that the Rays have been good more often than not, but have never really won anything (and the Orioles have struggled to get over the hump the last few years).