Right, there are several but to name a few: the salary cap, free agency and a much more competitive & improved scouting system league wide. If a team was able to assemble a line that was equal to or better than the line we had in the 90's, it would only be a matter of time before they were broken apart due to their salary demands. If the GM put the $$ to keep a line like that together, the rest of the team would be crippled.
I wish they would adjust the salary cap to give the more successful teams (in terms of finances) a % of their gross revenues as additional cap room. This would reward the more successful franchises and perhaps exert pressure on those franchises that don't pull their weight. At least this way, we'd get something out of what Jerry brings to the table.
It would be easier to replace 1 player on a great OL with a draft pick than it would be to replace 1 player on a bad OL. For example, if Leary were added to an OL with 4 great players, they would be able to have the Center help him the majority of the time; however, in the current Cowboys situation, despite the fact that Leary had zero experience prior to this year, they have the Center helping the RG the majority of the time. This makes the degree of difficultly much higher for Leary.
If a team had 5 great OLinemen, they would need to replace 1 with a draft pick about every other year in order to have a reasonable salary cap expenditure on the entire OLine. Draft picks are cheap for 4 years (now 5 for 1st round picks).
There are high upside players in the mid-rounds that usually are just not quite ready to play as a rookie. A team should probably draft an OLineman every year somewhere between the 1st and 4th round. In order to maintain a great OL, they should probably focus on high upside players that just need time to develop and are available later due to issues like going to a small school. Erik Wiliams went to Central Ohio State and Larry Allen went to Sanoma State.
I think the Cowboys might be starting to do something similar to what I'm suggesting. Physically, players like Leary, Weems and Parnell had high upside when acquired. Obviously, Frederick and Tyron Smith are high upside players. On the flip side, players like David Arkin, Phil Costa, Kevin Kowalski were not high upside players, IMO.
The problem for the Cowboys this off-season is that they need 1 more Guard that has the ability to step in and start in 2014. They have zero depth at that position and you can't count on a 3rd round draft pick being ready to play as a rookie. IMO, that indicates that they need to get a Guard in the 1st or 2nd round or sign a free agent.