Forcing people to adopt a language different from their own cherished one is something that has a strong tendency to backfire violently and quickly in modern times, as places like Flanders and Catalonia show. Even more so in Eastern Europe. Slovenians had long fought for their language within the Austrian state against the imposition of either German and Italian. Heck, the use of Slovenian in some local schools was among the hottest political issues of the whole Hapbsurg Monarchy at the turn of the centuries. Slovenian is indisputably close to Serbo-Croatian, but it was an established literary language from before Jugoslavia even formed. Forcing Serbo-Croatian upon Slovenians would only encourage Slovenia to seek independence.
I tend to agree, but I do think that Yugoslavia could have created a seperate language to unify the various languages in the country. To be honest, Serbian, Croatian, and Bosnian are really the same language. Macedonian is really Bulgarian which is very close to Serbo-Croatian. And Slovenian is also understandable to speakers of Serbo-Croatian. Right now, everyone is trying to tear the three languages apart, but a movement could have created a generic, gulf-arabic version of the language. Not sure if it would have helped keep the country together though. I think if Slovakia and the CR couldn't work it out, Yugoslavia couldn't. Especially since Slovenia (and to a lesser extent Croatia) could clearly see that they would be much better off economically without the drain of the poorer southern republics. Had Yugoslavia tried to stay socialist, Slovenia and Croatia would be unhappy since they would be very sucessful as capitalist countries in Europe. In that scenario, the West would be helping them get free from Yugoslavia. Had Yugoslavia tried to embrace capitalism, it would have led to the natural growing pains of switching to a market economy that Slovenia and Croatia didn't really have to endure since they were better suited to join the west. In that scenario, Slovenia would be looking to bolt since they could go it alone and be a thriving economy in a year or two as opposed to being part of a broken Eastern European economy.
And as we saw in OTL, Yugoslavia in the 1990s embraced a broken form of socialism with just enough capitalism to screw the economy ever worse than if they left it alone. that led to hyper-inflation. No way Croatia or Slovenia would tolerate going through that. I think the only way Yugoslavia stays together is if Slovenia is alowed to bolt. Maybe (and a big maybe here) the rest of the union could survive in a loose confederation. Or if something happens in the early 1980s where a Serbian is able to take over the country and claim to want to rid Yugoslavia of communism (while also destroying Tito's multi-republic structure). The west might be willing to flood money into Yugoslavia then (the USA sees a cold war victory and floods the Serb government with money) and most westerners then would not really understand the significance of getting rid of the Yugoslavian republics in 1982 or 1983. But that scenario is almost ASB.