They can't sign both, that is just out of the question. You would be pushing $70mil or so committed to 3 guys.
You would be right the prospects have proved nothing, but they are the future and our success pretty much depends on those guys. If they fail no amount of keeping Woodruff/Adames will even matter. This is no different than years ago when our future success was hinging on the likes of Peralta/Woodruff/Burnes to break out and stay that way. So yah, the prospects may all flop...but at that point nothing will matter much as we will be in a full blown rebuild.
Adames is an elite player, I am not debating that. However, offensively, he is not elite purely just looking at his offensive stats. Adames is at risk of finishing the season with an OBP under .300. He just isn't a massive force in the lineup. It is even more notable when you consider his position. If his defense declines or he switches positions...he just becomes that much less valuable. We just watched Wong's defense go into a dumpster at 30-31. No one really wants Wong for one year at under $10mil...then we should have no business signing Adames to $100mil+ that will largely cover the years we just had Wong at.
We only have 5 pitching prospects in our Top 30 from MLB.com. Two at AAA that are back of the rotation type guys, one is a bullpen flamethrower, and the other two are at #19 and #20 in A+ ball. Houser/Lauer are getting expensive and only have two years of control remaining. We simply don't have the reinforcements in the minors to fill all those rotational spots. So you would have to go spend Chacin/Garza type money to hopefully go find a #3 starter to compliment Peralta/Ashby. If you think about it, if we have to go the FA route, those guys are going to be just as old as Woodruff.