This is impractical already and it's happening: Catalonia wants to secede, but no one has any clue of what's happening afterwards: They'd want to become a separate nation. This nation would require to be recognized. If the Catalan nation would actually be recognized, it would not be part of the EU, thus have (at least initially) border checks, tariffs, no freedom of movement. It probably could keep the Euro as de-facto currency but have no power over its policy. It could apply to become a member of the EU, but a single veto of an existing member would block that. Now, which member might actually go veto a catalanian EU-membership? I can't possibly think of any, maybe you can. Even if not vetoed, this process takes years. What happens to people that live in Catalonia, but self-identify as spanish? What happens to people that live outside Catalonia, but self-identify as Catalonian? Their pensions, their work and residence permits? Their families and livelihoods? Shouldn't they be allowed to vote as well?

To be extra-clear: I don't think the hard-line course that the spanish government is pursuing is good, but I don't consider the forceful push for a referendum against spanish law and constitution is helping either.

So yeah, impractical, idiotic things happen all the time and the minority (in your proposal up to 1/3rd plus quite a few) is dragged into the mess. These things must be negotiated carefully and consider what will happen after secession. A referendum on a full treaty could work, see the separation of Czechoslovakia, but an unilateral declaration is just a mess. (see also: Brexit - where Theresa May just called upon support from the worlds leaders because of tariffs imposed by the US on Bombardier airplanes. If only you could be member of a major, powerful trading block.)