It wouldn’t last long. When I left El Salvador, the gangs outnumbered the military. But that doesn’t translate into war-making capabilities— only instability, which they over-indulged since the last Mano Duro II operation that was short-lived.
Also, all the gang members are tatted up, unable to hide like the guerrillas did in the El Salvadorian civil war. They are loathed by mainstream society, so there is no where to hide in a small country.
Lastly, both conservative and liberals abandoned their mainstream parties (ARENA and FMLN) to elect an outsider because neither of them did anything except submarine each other in the General Assembly to prevent progress in public safety.
Back when the guerrilla war was ending, the most famous maras were the Mara Chancleta and the Mara Gallo and neither was that big of a deal. The “peace process” eliminated the Policia de Hacienda, the Guardia Nacional, and the Policia Nacional which created a law enforcement void that the new civilian police were unable to fill. Yes there was some corruption with the para military groups, especially the plain clothes units but they also had kick ass capable units. The weaker enforcement led to the big gangs like thevMS13.