Pretty much the whole of Latin America (or at least certainly South America) from the 1960s to the 1980s was run by military regimes of one sort or another. Certainly Peru, Argentina and Brazil were beginning in the 1960s, Chile and Uruguay were beginning in the 1970s, etc. Spain was a one-party state of sorts from the 1930s to the 1970s, and Mexico was a one-party state until the 1990s. The transition model in these countries has been one in which 'democratic' reforms are gradually introduced, and the old regime is left a way out that doesn't involve arrests, tribunals, and possibly executions. Typically the old regime holds "free" elections (typically starting at the local/regional level before making their way to the national/presidential level) in which the regime's party competes with the opposition. The old regime makes constitutional changes which legalizes the opposition while giving the remnants of the old regime a way to keep some of their power. In short, they give the leadership of the old regime a stake in the new society. If you give them a stake, they would want seek act as a militarized opposition to the new government once the 'revolution' takes power.
The only case in which a complete toppling of the old government might work is if the opposition is organized and coordinated enough to govern. In all of the cases the opposition is highly fragmented and disordered. There's no real cohesive, centralized leadership with military discipline and a coherent program. With fragmented, disorganized opposition "revolution" is a recipe for complete disaster.