Which separatist leaders are you referring to? After the fall of communism in Eastern Europe, Yugoslavia's split was imminent. Separatist forces were strong and widespread. There's only so much he could have done.
To begin with the leaders of the separatist movement in Kosovo.
In Croatia and Bosnia and Herzegovina separatist movements were also present. Milosevic gave them time to organize an army and to expel the Serbs. The ethnic cleansing of Serbs in 1991 in Croatia took place in front of the Yugoslav army.
Milosevic and the presidency of SFRJ could arrest Tudjman in 1990, when Tudjman violated the constitution. A recognized state is authorized to end separatism within its recognized borders.
The USSR indeed collapsed in 1989, but do not see why Yugoslavia should also collapse then?
If he had not surrendered when he did, NATO would probably have started carpet-bombing. In hindsight, it would probably have been better if he had just accepted the Rambouillet agreement rather than let the country get shred for 78 days. Same outcome except no senseless deaths and destruction from NATO.
The Rambouillet agreement was actually an ultimatum, according to which Serbia should allow NATO troops and police to enter in whole Republic of Serbia. Allowing foreign forces to enter on your whole territory is surrendering your sovereignty. Only with that term of the Rambouillet agreement Slobodan Milosevic did not agree. With all other term he agreed. The Rambouillet agreement is to be compared to the agreement proposed by Austria in 1914 and Hitler in 1941.
Before declining the Rambouillet ultimatum, Milosevic held a referendum in Serbia.
The Serbs could chose between protecting the Serbian territorial integrity and sovereignty in Kosovo or allowing NATO troops to occupy Kosovo. The people voted for option one. So Milosevic was right when he rejected that ultimatum offered by the NATO.
In the beginning of the NATO terror campaign on Serbia the majority of the world supported that. At the end many countries did not support it just as in the beginning. Some sources claim that if Milosevic continued just a couple of more weeks the NATO would stop with the operation. Not all NATO nations wanted to totally bomb Serbia. Milosevic did not persevere so he betrayed his country!
The American war against Serbia in 1999 was not according to the constitution.
So I believe that if Milosevic just continued, NATO would stop with their strikes.
Some sources claim that.