Baseball hasn't been the most popular sport in the US in decades. Calling it the national pastime is one of those "that's the way it's always been done" things you seem to hate.
That's a common aphorism. I've never heard anything else called the national pastime, but that's not "doing" something.
I don't know what's "most popular", or how one would measure that, but the OP asked what the "national sport" is, not what's the "most popular". Anyway I don't think the fact that ESPN is obsessed with it makes it the "most popular" anyway.
Having no idea what I'd find I looked up Major League Baseball and National Football League total attendance figures for the last available year, 2017. Pro sports only, understanding these do not represent "all" of the sports.
A bit over 17.2 million people attended NFL games last year, while
a bit over 72.6 million attended MLB games.
So I'm still a bit unclear where y'all get this idea that "football has eclipsed baseball decades ago".
Even ignoring things like revenue and television viewership, baseball has 100x as many games as football, so they get quite a bit fewer people at each game based on those numbers.
Actually it's ten times. Even I can do that math. 162 vs. 16, almost exactly ten.
Baseball has a longer season, but then again it's arguable that if football were the "national sport", IT would have the longer season. They'd be playing into January and February and the Stupor Bowl would coincide with Spring Training. And baseball has been stretched to such an early point in the year, if football predominated MLB could wait for better weather, as it used to.
It's a nebulous concept that has no concrete answer but I was just looking for some basis for this belief that "it used to be baseball, now it's football". I didn't find one.
LOL, I sure did add an extra 0.
Football has the most watched event, the Super Bowl, by far. Football has the most revenues, although baseball does surprisingly well. Top Professional Sports Leagues by Revenue
In 2016, the top 10 most watch sports events in the US were all NFL games. What The 10 Most Viewed Sports Events Tells Us About Our Bond With The NFL, NBA And College Sports
The most money brought in, the most people watching per game, both in the stands and on television, the most watched sports event in the country every year for years. Football is clearly more popular than the other major sports in the US by most metrics.
On the other hand, if you want to judge national pastime by what game people play most often, that's different. It might be baseball, although Wiki (the only place I found numbers in a quick search) puts basketball slightly ahead of baseball: Sports in the United States - Wikipedia
Baseball is still surprisingly popular, but it is not at the level of the NFL as far as professional sports go.
We are dealing with a nebulous definition -- the "national sport". There really isn't an official such title. But the closest match know of is the "national pastime", which has always been attached to baseball.
Again that's not the same question as "most popular", which is also vague and undefinable. Does it mean how many games are watched? How could we even measure that? My comparative numbers were between NFL and MLB only -- that doesn't figure in MiLB, college football, semipro leagues, Little League, or the aforementioned informal neighborhood kids squaring off of an afternoon in any sport. They all count toward 'popularity' as soon as people engage in it.
Or does it mean how many people play at any level of a given sport? No way to know that either.
And to turn the disparity between MLB and NFL games on the other side of its coin, baseball is played every day of the week including traditional workdays while football is reserved for Sundays, holidays and only an occasional evening, which sets it up with an availability advantage.
Or does it perhaps mean to what degree a given game is understood enough to follow with interest? The traditional father and son playing catch?
And does the fact that baseball is also popular in Latin America and Asia while (American) football is not, tilt the scale to football? Or is baseball "our" sport that we let "them" play? It's always a bit disingenuous to call the last stage the "World Series" when there are no teams from Cuba or Japan...
No real answer because many answers.