Tim Anderson would be the best but team option will keep him off the market. Crawford is almay also there but older. And, Ushella never gets listed but will most likely be starting SS for Angels and has better glove than Rosario even if older. Machado may hit the market (but at $30 million 5-year reamining player opt-out it is less than 50/50) but Baez (with an opt-out and no SS market on a losing team) may want to up his 4-yr $25 million remaining contract.
Even though Rosario will be the best "SS" on the market as all others have been locked up long-term, there will be other options of converting back one of these other guys (Urshella or Machado) or getting a older Crawford for a desperate team. Thus, Rosario may get $15 million for a few years on the market without any tags but when a team loses a pick by signing a "tendered" guy, it makes them hesitant to go all in when there maybe other options (like trading with Yankees/Rays for the SS that doesn't work out in their system) or coming after Guardians when Gimenez refuses to resign and 3-years left (but it would take a astronomical haul to do it next off-season).
Good thought and if Rosario has a break-out year, it may happen. But as discussed here before, Rosario doesn't have the power or plate discipline to walk enough to get those $20 million offers no matter how weak the market is; otherwise, the trade market now would be tempting enough for the Guardians to pull the trigger and go with Arias or Freeman.