There are plenty of cases of players who miss time due to injuries or personal issues (Josh Hamilton, Jayson Werth) and continue down the development path they were expected to have, but here lies the key to the issue, continue their development path, where is the indication that Lowrie's development path was going to take to 20-homer, 40-double land?
Sure, stranger things have happened, (See: Ramirez, Hanley; Cano, Robinson, Youkilis, Kevin) but is that a reason to say that something as unlikely as him hitting 20 homers or slugging .520 is a near certainty? Because that is being a homer of the worst kind. Now, i'm not saying you're doing that, but let's also mention that Beltre's 2010 season was a fluke indeed but it was not close (power-wise) to 2004. He hit 48 homers and had an OPS over 1.000 , 2010 ain't even close.
Also, food for thought, here are the players who slugged .520 or better last year: Corey Hart, David Ortiz, Matt Holliday, Jayson Werth, Robinson Cano, Luke Scott, Adam Dunn, Beltre, Tulo, Paul Konerko, Albert Pujols, Carlos Gonzales, Joey Votto, Miguel Bautista, Miguel Cabrera, Josh Hamilton.
The 20-Homer list ain't too long either.
My point is that being optimistic and realistic are different things. Hell, he could break out and hit 54 homers like Bautista, but come on.