Personally, I've found Mastadon to be a fantastic replacement. What do you believe are the things that make Mastadon not a decent replacement? It definitely fits better with the public and open service that people wanted Twitter to be.
Nobody uses it. Talking to yourself was not a mainstream use-case for Twitter users.
Who is nobody? Almost all of the people I used to follow and discuss the topics I cared about on Twitter are now all on Mastadon. I find that I'm having similar discussions at this point with almost all the same people I used to.
I do miss quote-tweets as a mechanic; while they could certainly be abused, there were legitimate use cases. Mastodon plans to add them, but AIUI getting the appropriate changes made to ActivityPub to allow for consent-based quote-posting is a whole thing.
I’m not sure that there will ultimately be _one_ Twitter replacement; with the exception of Digg->Reddit, it’s actually rather unusual for that to happen when a social media site dies. Livejournalers-in-exile mostly didn’t go to Dreamwidth, say; they scattered to a bunch of rather dissimilar social networks.