I would consider keeping an Iconic Brand alive, and spawning (for better or worse) a host of Third Party Publishers (many that still exist to this day)to be Noteworthy, yes, even if they had produced half as many books and it made only 1% compared to MtG.
Your take on it seems kind of flippant, like D&D was just a side-project that no one took seriously since churning out MtG cards was where the paycheck was.
Also, since I'm not in the 1%, I consider making about 6 million a year pretty good off of some insignificant books.
Let's not forget that WotC, in the 90's, was notorious for buying the competition. Their RPG's were (largely) poor sellers. Several were acquired as part of purchases of other companies. Most of the purchases appeared only to be pursuant to shutting down the competition.
Sufficiently so common that buying TSR was almost blocked on anti-trust grounds. Keeping TSR going was part of the settlement with US DOJ. Since HasBro bought WotC, they seem to be honoring that deal.
Long run, it's probably cheaper to keep printing D&D (especially for the spinoff IPs) and keep the design team happy than to kill it as non-productive.
In and of itself, D&D as a brand is minor for WotC, trivial for HasBro. The spin-offs (movies, novels, videogames) raise to notable for WotC and barely noticed for HasBro.
about 15 people at WotC work on producing books, counting one in Accounts Payable, and one in Accounts Receivable, and one in payroll. And the ones in accounting and payroll probably aren't devoted full time.
Less than 1% of the income, less than 10% of the staff, but generating millions in secondary licensure. (probably a good bit more than the 10% of staff would generate if working on cards or boardgames.)
Those D&D staff are in the business of making a brand worthwhile financially - not in the business of making books for the sake of making books, nor of making games for the sake of games. The books are, at best, a means to an end. They're really part of the advertizing for the novels and computer games. The computer games are the apparently largest share... but they feed off of the RPG team doing story. Likewise the novels crossfeed and cross-fertilize.
The books are, essentially, just advertizing that pays for itself.