I would be just as content with Cordy Glenn in round 1, and I like the guards in round 2. An offensive line pick, in round one, is generally considered a solid, safe investment. (see: Tyron Smith)
Here is why some fans are concerned about the guard position in general and see DeCastro as a huge upgrade:
David Arkin (4th round) couldn't even dress last year, and he is hope and not a plan at this point. He couldn't even beat out Nagy. DeCastro (1st) and Cordy Glenn (1st) and Kelechi Osemele and Silatolu and Brandon Washington are all more talented players and better prospects at guard.
Since when should a re-building team (on the fly) ignore that type of talent due to a developmental player like Arkin? Dallas' QB and RBs need guards now. Why use cheap building materials to build onto your 21 year old, blue-chip cornerstone, Tyron Smith (# 9 pick). Why not go for a dominant offensive line in order to counter the Giants and Eagles and Commanders? This draft is aligning pretty good with Dallas' needs at OG and CB, too.
Bill Nagy (7th round) was really undersized at guard and lacked power in the interior running game. PFF graded him the worst run blocking OG while he was playing. The Cowboys replaced him with Dockery and Holland for a reason. His best position is probably center. Nagy was a backup player at Wisconsin after he was injured off the field.
Kowalski (UDFA) is an undersized center who is unproven and lacks size and power. He did a decent to good job in a limited role as a backup, but his run blocking was not nearly on par with Costa's run blocking.
Montrae Holland was cut and re-signed during the middle of the season. He had a nice little run, but he was hurt against the Eagles and was lost. Holland was a little bit of lighting in a bottle and counting on that again would be foolish. Holland has pretty much been a backup player since leaving Denver.
Everyone wants to run Phil Costa out of town, but he was a decent run blocker. He's undersized, too.
The Cowboys are in a re-building phase, and they have no business ignoring the top guard or center talent in this draft class.
I see a whole lot of hope and not much of a plan.