At least here in California, John, the majority of people who picked up the dulcimer in the early 70s did so because of Joni Mitchell and Richard Fariña.  There was no pre-existing dulcimer tradition out here until people heard those two pop stars.  And many of those people are still around, playing and teaching others.

When I got my first dulcimer I was employed teaching beginning guitar to some middle- and high-school students.  I brought my dulcimer in to show them my new toy, and they all got excited because they had just seen Cyndi Lauper play one on TV the night before. But by then, Lauper was old news.  When she had her hits back in the 1980s, she never appeared with a dulcimer (nor any instrument, for that matter).  Had she done so, perhaps I would have discovered the dulcimer decades earlier than I did.

Hey now Dusty, the Nate fanclub may be small, but it has a quality few members. xD

It's a very true point though. Popular artists of the 70s played a huge role in the re-emergence of the instrument. Joni Mitchell, Emmylou Harris, Led Zeppelin, Rolling Stones, etc. Often when you look at the comment section of tracks from that era that included dulcimer you will see people talking about how the sound stood out to them.

In modern times, Harry Styles, who was a member of the popular boy band One Direction, had a song that heavily featured the dulcimer and while his audience probably didnt notice, as a dulcimer player I definitely did.

Has the dulcimer playing of artists such as Brian Jones or Joni Mitchell or Cyndi Lauper or Dolly Parton made a measurable difference?  I don't know.

Maybe Ms. Swift should consider playing a dulcimer, if she could only sit down for two minutes in her concerts.  Attention would certainly be drawn to the dulcimer in such a situation.

Interesting question.  I'm not sure how to measure it, either.  The dulcimer was never a popular instrument. Even in Appalachia in the 19th century, it was less common, surely, than fiddles, banjos, and guitars.  The folk revival did indeed represent an apotheosis of sorts, but importantly, a lot of the people who took up the dulcimer then are still involved in the instrument. I've learned from many of them (Neal Hellman, Joellen Lapidus, Holly Tannen, etc.)

The original question references only the last 10-15 years.  Since I first discovered the dulcimer 13-14 years ago, I think my own perspective is useful here.

Until the pandemic, the number of dulcimer festivals was increasing.  And throughout that period, the number of people engaging about dulcimers online (FOTDM, ED, Facebook, etc.) has also been increasing.  In terms of the sheer number of people who own or play the instrument, I think there are more people involved now than there ever were.

And I'm not sure that the number of "commercial" builders is a measure of an instrument's popularity. First, as @Nate says, the dulcimer is relatively easy to build, and a lot of people build dulcimers for themselves or close friends. Second, and I will argue this until the cows come home and crush our tuners, we now have a small number of phenomenal luthiers making instruments that far surpass in quality of tone and playability the vast majority of instruments made in the past.  Even long-standing "commercial" luthiers such as Folkcraft and McSpadden are making much better instruments than they did decades ago, and if you add New Harmony and David Beede and Ron Ewing and Jerry Rockwell and Terry McCafferty and so many more, I think the number of outstanding dulcimers being produced is greater than ever.  Makers of some less impressive instruments have ceased, but I think that's not a bad thing.  Additionally, the market has been sufficient for low-end commercial builders like Roosbeck to enter the fray.

Having said all this, imagine if Taylor Swift would just play one song on the dulcimer in one of her concerts, I bet a whole bunch of teenage girls would get turned onto the instrument and before you can pick up your noter, there would be a NateBuildsToys Fan Club formed, their insignia a jello mold in the shape of a treble clef.

While I totally agree with you Wally, I think that dulcimer has recently rediscovered it's place as an instrument that is great for amateur builders to make for themselves. I have personally taught a bunch of people to build dulcimers. With modern tools and hardware, it's easier than ever for people to make their own dulcimers, and for people who like to build instruments, dulcimers and strumsticks are a common project.
Personally, I mainly built cigar box guitars before learning about dulcimers, and I have preferred making dulcimers since.
I think that the lack of commercial viability is also an indication of the durability or dulcimers, and the care given to them. There are probably more dulcimers out there than dulcimer players, so making new ones is best left to folks who make exceptional instruments, like many of the fine luthiers on this site.

One measure might be the number of commercial producers of dulcimers. That has certainly declined.

Certainly not my interest! I really want to know what folks who have been playing for decades think about this.
I am a young person and have not played for very long so this observation is limited. From my conversations with others, it seems that the instrument has been on a slow and steady decline since the folk revival. I think the instrument has incredible potential for a come back, but at the moment I would say that the popularity sadly seems to be on decline.

Has the popularity of and interest in the mountain dulcimer declined in the last 10 or 15 years?