There is a small cohort of people, 40-48 years old, who experienced the ideal of instant messengers in the late 90s to circa 2005.
After that, everything in the unified/personal communications space got technically better but worse.
After that, everything in the unified/personal communications space got technically better but worse.
Circa 1999-2000, you could IM random ICQ users and they were not 100% dickpic creeps. It was almost like a pre-AOL internet for people in the know.
That's an entire generation!
I agree that classic IM was best IM, though. Unfortunately you can't make money on IM alone, and AOL isn't the portal of choice anymore. (Google was so close with Hangouts!)
I can't say that an unbloated app like Signal is a fundamentally different experience except for some of the deficiencies that've been ironed out (better file/image sharing is one that certainly comes to mind).
I'm open to being wrong on this point though!
EDIT: Actually, you can add Discord/Signal/Telegram to Pidgin for the throwback UI.