Artifacts are great. They solve the issue of having to wait on a good weapon drop before you become effective, and they, essentially, act as Alternate Advancement. I'm not even sure why there's a complaint that they "never" end. Once you get Concordance, they're, essentially, finished. All AP into Concordance after the first point are very minor bonuses and, frankly, a good compromise between "never" ending and being completely finished. Now, we have the Netherlight Crucible coming in 7.3, but that will, essentially, be finished in a couple of months when we have AK50 and hitting Artifact Level 75 isn't the insurmountable climb it currently is.
TL;DR: I disagree about Artifacts, and I hope we get new Artifacts next expansion.
Legendaries being entirely RNG was a mistake, though. Some specs rely heavily on specific Legendaries that make the current system outrageously frustrating, and that's to say nothing of switching specs or, worse, classes. Furthermore, throughput Legendaries are, clearly, much more valued than utility Legendaries, and that's a serious design flaw.
I'm fine with us working towards getting multiple Legendaries in future expansions, but they should be built like many of the old school Legendaries. A grind with a known reward at the end is a superior system then just relying entirely on luck (And the Relinquished "targeting" is rubbish).