An argument as weak as a stack of Jenga blocks
Your argument has a hundred tangents, and none carried to their logical conclusion.
Yes, the USSR fell apart, and as you wrote,
Some countries like the Baltic States prospered, while others withered away into insignificance like Tajikistan or civil turmoil like the Ukraine. Is this the best future for Catalonia or Spain?
Excuse me? Is your argument that the Baltic states should not have abandoned the USSR? And elsewhere, when you discuss the collapse of Yugoslavia, are you making the same case, there? That Yogoslavia should still exist as a country, with Serbia, Montenegro, Bosnia-Herzegovina, Croatia, Slovenia, Macedonia, and now, Kosovo, bottled up as one country, despite the desire of most of those peoples for self-determination? Should we rewind to the borders of the Austro-Hungarian Empire?
While the central government of Spain may want to control the desires of the Catalans, Basques, and other peoples now incorporated in the remnants of the Spanish empire, that doesn’t mean they are morally justified in suppressing the will of the incorporated.
And, as usual, the question behind this has to be, who owns the future? Do the residents of Catalonia — and more pointedly, those whose primary identity and affiliation is with Catalan culture and history — get a say in their own future?
The judgment of whether creation of an independent Catalonia and the potential for a collapse of Spain is a good idea can’t be weighed by analogy with the breakup of Yugoslavia and the USSR. It should only be weighed by what the Catalan people want.
The UN Charter’s article 1514 says a people’s right to self-determination shall not be suppressed, despite what the UN might say today about Catalan independence.