That's definitely part of it too, but it can't be denied that Bjj fills Judo's void almost completely. One example would be the interest in No Gi Judo that occurred after the rise of Rhonda Rousey in MMA. Where could you go to learn No-Gi Judo? Not at a Judo dojo, you had to learn it at a Bjj gym.
Bjj has a knack to eclipse and then slowly absorb any rival grappling system that emerges to challenge it. For example, Sambo and Catch Wrestling both attempted to make in-roads into the grappling scene earlier in the decade. Instead of ignoring the challenge, Bjj instructors wisely began bringing Sambo and Catch Wrestling instructors into their gyms to teach their students. Before you knew it, Bjj guys were doing leg locks and Catch Wrestling holds all over the competition circuit, effectively making the learning of either of those other grappling systems superfluous. Bjj is now starting to absorb Judo as well.
You are correct though. I think a big problem Judo has is that Tachi Waza is a lot harder to learn than Ne Waza, and frankly wrestling's takedowns are easier to learn and combine with Bjj and MMA than Judo's throws are. Unfortunately, Judo instructors frown on you bringing wrestling takedowns into Judo.
I'm not sure it's Less popular than it ever was, as it was never particularly popular,
But your last point applies, to most tmas, not just judo, as soon as the T appeared in tma, the arts were signing their own death warrant, as it's a declaration, that they won't change adapted absorb, the arts evolved to a point, where they were then frozen in time and if you don't like it, so something else.
Bjj, is the English language of ma, it just absorbs, and discards, judo and other tmas are the French of ma, maintaining its cultural heritage is far more important than, increasing it's application Or it's US e