There are many ways of talking about "the canon" without actually saying those words. For example, yesterday I listened to Mahler's Symphony No. 3 which is over 90 minutes long. The notes to the recording mentioned that it was the "longest symphony in the standard repertoire". This is another way of referring to the canon: standard repertoire, that is, the collection of pieces that most orchestras are likely to play at some point.
This is a larger repertoire than what we might call the "popular" symphonies, though that is a kind of synonym for "the canon" as well. The popular symphonies are those ten or fifteen or forty or fifty pieces that most symphony-goers would recognize immediately: Schubert's "Unfinished", Beethoven's Fifth, Haydn's "Surprise", Mozart's "Jupiter", Tchaikovsky's "Pathétique". Often, but not always, these popular symphonies acquire nicknames. Mozart's 40th is just as popular as his 41st, but has no nickname.
CORRECTION: I actually wrote that only the 9th Symphony of Beethoven had acquired a nickname, completely forgetting about the Eroica and Pastoral. Luckily a reader corrected me!
But, as I was saying, the standard repertoire is larger than just the really popular symphonies and includes pieces that are played pretty often and enjoyed but that have a modest profile in the minds of even concert-goers. I am thinking of pieces like all those wonderful Haydn symphonies that don't have nicknames, the earlier Schubert symphonies, the even-numbered Beethoven symphonies like No. 2 and No. 4 that are played a lot less often.
But then there is another, more obscure category that I'm not sure how we should name: pieces that are certainly part of the repertoire, but are far from being "standard". Pieces, in other words, that are rarely played and only known by and popular with a small percentage of listeners and performers. Perhaps they are pieces that are of merely historical interest, like the symphonies of J. C. Bach. Or perhaps they are just slowly edging their way into some version of the canon like the symphonies of Allan Pettersson. Or perhaps they are just too new like the symphonies of Philip Glass or Peter Maxwell Davies.
What do we need the idea of a canon for, anyway? I think my sketch of the different kinds of canons gives us a clue. There is a functional purpose for each of the different kinds of canons. If you are a music director planning out an orchestral season you want to include some real audience draws: some of the really popular symphonies. But you don't want to use the same ones you did last year. So over the years there accumulates a list of symphonies that are reliably popular with the people that buy season's tickets. That list is constantly changing, though, as symphonies do go in and out of fashion, at least some do. Then there is the other list of symphonies that you include to balance or fill out a program or to lend variety. "We can't keep flogging the same Beethoven symphonies every year, this year let's do a few by Sibelius." So that is how the standard repertoire develops.
Despite what I said about about scholars trying to "cultural theory" the whole idea of a canon out of business, they too play an important role. Some scholars are involved in re-evaluating the worth of parts of the repertoire and how they do this is by giving some works more prominence in their histories or by writing monographs on the composers. Some scholars are trying to educate both musicians and listeners about composers they feel deserve a wider exposure.
So there is a kind of constant "Brownian motion" of symphonies rising and falling in terms of popularity measured by number of performances and recordings.
And apart from mentioning that sometimes the use of a work or more likely a part of a work in a film or television show can give a piece of music a real boost, that pretty much covers what I wanted to say about the canon.
Here is a piece that got a big boost in popularity from being used in a Swedish film called "Elvira Madigan". It's not a symphony, though, it is the Piano Concerto No. 21 in C major by Mozart: