OTP is the core framework and main value proposition for both Elixir and Erlang. It’s so core it’s part of the kernel. Without OTP Elixir probably doesn’t exist and Erlang is just a weird Prolog variant nobody remembers. It is to Elixir what the DOM is to JavaScript.
On the sidebar: Languages have their purposes and being a pure functional language reference design is not one of Elixir’s. Characterizing Elixir’s diff of Haskell features as a negative or con is misguided and a disservice. Both are functional. Haskell exists to be purely functional at the expense of pragmatism. Elixir exists to be robust, fault tolerant and concurrent at the expense of purity. Being a functional language is a means to an end in Elixir and Erlang, it’s not the reason they exist unlike Haskell. Some of the things you list as negatives are impossible without a static, strongly enforced type system. Elixir has a dynamic, weakly enforced type system.
