On the surface that argument makes sense, but dig a little deeper. The number of teams that make the playoffs has changed dramatically over the years. As a result, comparing number of playoff appearances and playoff won-loss records over time is a comparison of apples to oranges. Good teams may rarely miss the playoffs today, but that was not always the case.
There was a time that it was not the least bit unusual for a team that won twice as many games as they lost (or better), or a four-loss team to miss the playoffs entirely. Today on the other hand it is not the least bit unusual for a seven-loss team (or worse) to make the playoffs, while teams with losing records don't get eliminated until the final game of the regular season.
We have gone from the playoffs consisting of one game to two, to as many as four for the championship team. The expanded format means that the total number of wins will obviously be more now. For older teams the improved competition due to their being fewer teams in the post-season makes their cumulative won-loss winning percentage appear to be mediocre. A more accurate comparison would be a current teams' W-L record in the Super Bowl, or in conference championship games plus the Super Bowl, or comparing playoff appearance from past years to current appearances in a conference championship game.
I did dig deeper when in a previous post I compared the pre Kraft Pats to examples of teams that I was told were worse. Only 3 out of 7 of the teams used were clearly worse than the Pats, and 3 were clearly better. It's not apples to oranges when you are comparing the exact same years for each team. Now that it's been established that it's an apples to apples comparison, I shall address the rest of your argument.
There are two problems with using "the record of teams that missed the playoffs" as a metric. First, there is a metric that measures exactly how hard it is for teams to get into the playoffs; total number of teams/teams that make the playoffs. Second, there was much less parity before the days of the salary cap and free agency. In other words, more teams had really good or really bad records vs. .500 ish records than in today's game.
Here is a grid showing the proper metric for determining how hard it is to get into the playoffs after each expansion and format change. I left of before the merger because the # of teams differed between the AFL and NFL.
Timeframe Total Teams Playoff teams Total/Playoff
2002 expansion 32 12 2.67
1999 expansion 31 12 2.58
1995 expansion 30 12 2.50
1990 format change 28 12 2.33
1978 expansion 28 10 2.80
1976 expansion 28 8 3.50
1970 Merger 26 8 3.25
Although it was more difficult to get into the playoffs until 1990, it was never greater than 1 in 3.5. Less teams in the playoffs and less playoff games certainly affects the number of playoff games played, but it has no bearing on Win Percentage. Not only that, less teams in the league means a team should make the superbowl more often.
Even though it was siginificantly easier to get into the playoffs after 1989, only 3 years (9%) of the timeframe I cited took place during that period. On average during the previous period a team should have made it into the playoffs more than once every 3 years. The Patriots (Who weren't in the playoffs from 1990-1993) were in the playoffs less than once every 6 years. No matter how you slice it, before Robert Kraft purchased the team in 1994, the Patriots fielded a crappy team more often than not.
Sources:
National Football League playoffs - Wikipedia, the free encyclopedia
Timeline of the National Football League - Wikipedia, the free encyclopedia