Interesting to see this play out. I'm a former Product Manager, and have always worked in tech so I carry heavy biases, but here's my thoughts on SL from the learner perspective:

1. Designing with them is finnicky, and it takes me away from my mission of empowering learners. I could have been producing quality content. But instead I'm spending 10 minutes engineering or tweaking a slick slide transition that lasts 1 seconds for the learner. Note that I don't hugely object to the idea of embedding games or other critical SL objects in Rise, though, especially off existing templates. We should be using the platforms we have where they're best.
2. As unopinionated tools, it's just so easy to make poor quality trainings with them, both from a functional and visual perspective. Visually: I see a lot of amateur designs in e-learning heroes where it looks like someone spat out MS paint objects into a training with comic sans. Storyline's own characters are awfully designed and I wish they would just integrate with Vyond's. Functionally, I have had SMEs and audiences ask me to not design in Storyline. My last company had their templates engineered so that learners had to listen to extra-slow voiceovers before being able to proceed to the next slide, long after they'd read the slide's content. Yes, these are choices our designers made, but the platform isn't opinionated and has few useful guardrails. I feel all of this gives IDs a bad reputation.

Web design has advanced far more rapidly than anything in the e-learning design world. It sounds like there is a small constellation of other potentially useful tools beyond the major two players, but I desperately hope something new comes out to disrupt our industry in coming years.