I'll get right to the point here: Tony Romo's at the age where people start to wonder how much longer he can keep things up. Additionally, there's some widespread belief that 2012 was a terrible year for Tony. I'll try to look at these two things with a simple chart. (Click on the button for enlarged version)
As you can see, we're taking a look at Tony Romo as compared to Peyton Manning. I've taken all of Romo's years as a starter and compiled a percentage breakdown of, essentially, 'what happens when Tony Romo gets the ball?' I included rushing statistics, as in most cases those are decisions to tuck the ball rather than designed runs. I could not readily include fumble data because it's unclear whether the fumbles occurred on sacks or rushes (or bad snaps), and therefore I would be unable to correctly adjust sack and rush totals.
The bar layout is fairly simple. I've combined all passing attempts, sacks and rushes into a play total. On each one of these plays, the outcome was one (and only one) of the possible outcomes in the key to the graphs. From bottom to top, the outcomes become increasingly undesirable; measuring how far from the bottom a certain color reaches is a good indicator of a quarterback's positive impact on games, per play, while measuring from the top is an indicator of negative plays. If you click the image, you can view the full-size version for more legibility. The color key, from top to bottom, reads "Interception, Sack, Incompletion (non-INT), Rush, Completion (non-TD), Rushing Touchdown, Touchdown."
I'm not asking you to think that Tony has comparable performance to Peyton Manning - by these measures he's clearly not quite there. We're, instead, concerning ourselves with Romo's 2012 performance relative to the rest of his career (and, while we're on it, Romo's overall career consistency), as well as looking for signs of decline due to age. The year's I've selected from Peyton Manning's career correspond to a time when he and Romo were the same age.
First of all, when you look at the chart, Tony's overall performance in 2012 was not out-of-character, exceptionally bad, or even remotely conspicuous. He threw a lot of interceptions - but he threw a lot of passes; his per-pass interception rate was in-line with his rates from 2006, 2007, 2008 and 2010. What was somewhat low is his touchdown rate. In fairness, his second-lowest rate, in 2009, came in an otherwise excellent season.
The problem with the perception is that many fans (at least those who take a very positive view of Romo) have been measuring him primarily based on his 2009 and 2011 performances. 2010, which is in-line, again, with his first three seasons, is viewed as an outlier due to the significant amount of time missed (and 2011 looking better than 2009 certainly contributed to that). Somewhat ironically, 2010 saw positive plays come at their highest rate of Romo's career; the downside is that a significant portion of his negative plays were interceptions rather than incompletions. It's quite possible that the 2013 Romo will be more like the 2012 version than the 2011 version, if only because that level of performance is now well-established in Romo's career.
The natural follow-up is to look at consistency. This is where Peyton Manning comes in. It's fair to look at, for example, the locations of the yellow bars (rushing attempts) on both quarterbacks' graphs and conclude that Peyton has both been more consistent and consistently better than Romo. On closer inspection, however, it becomes apparent that even Manning's numbers fluctuated within reason. The variation appears 'random' (and I mean that in an analytical sense, rather than truly random in the sense of completely without cause), and not an indication of any sort of trend, upwards or down. Any real arguments for or against Romo's consistency would be purely esoteric; we've likely seen the extent of his variance already.
Now, as for the question of potential decline with age, Peyton Manning becomes a more useful comparison. You see, with Manning, we can essentially look four years into the future (from this data set). In those three seasons (he sat out 2011), Manning's numbers remained fairly contained. He set a few career highs and career lows in both positive and negative categories, in each of those three seasons, but there was no overall trend. It's all just noise.
While you could argue that there's no provable argument for Romo continuing to play at a high level into his late thirties, the way Peyton Manning has (which is true - you can't prove that), there's also the counterargument that Romo has 'played' in four fewer seasons, courtesy of his age coming out of college as well as his years spent on the bench (also true). Manning has played in nearly twice as many games as Romo (and more than twice as many once you remove the 16+ games Romo appeared in without a pass attempt). Is it so far-fetched to expect Romo to eventually play in as many games (6-7 more seasons) as Manning?
Some have argued Romo may have as few as two years (or less!) remaining as an effective quarterback. I find that incredibly pessimistic. The precedent has been set. I see no reason to draft a rookie who will need to be extended before Romo even begins to think of retirement.
Seattle, Washington, and Indianapolis have proven the potential for the quarterback position to be plug-and-play, which, in my mind, is even more reason to wait as long as possible before looking for a new one. It's possible to win a Super Bowl any time your team makes the playoffs. It's possible to enter the playoffs with or without a top-tier quarterback. If the argument for having a "plan in place" is that you want to win the Super Bowl every single year, then, despite how improbable that is, it can't be said that not having a plan in place for Romo's departure prevents that.
In fact, since backup quarterbacks see far less playing time, and are less implementable as 'rotation players,' I would argue that a backup quarterback would do less to help this team than any other backup position, aside from kicker, punter, fullback and potentially offensive line. A defensive player, or a skill position player, is one that could produce without forcing a starter off the roster. If we successfully develop the rest of the team, we can afford to ride out Romo until he retires.