There is scope for more ecomerce stores in Argentina and bring in competition for mercado libre.
this has not proven to be true anywhere else- ebay, amazon, google, and similar companies hold effective monopolies in much bigger, much more potentially profitable markets globally.
the idea that you could overthrow an established brand like that is usually not proven in the real world.
the exceptions, like the recent rise of WeChat, in China, is due to special circumstances, usually government support. WeChat, for example, has the chinese government helping it in many ways, including integrating its payment system into the state controlled chinese banking system. It has 900 million users, and its only charge to them is .01 percent (one tenth of one percent) and only on payments.
I cannot imagine an argentine startup that would have the funding required to compete with mercado libre charging one tenth of a percent...
nor can I imagine the argentine government mandating effortless electronic transfers of money from all banks, either to mercado libre, or a competitor.
I think, in this market, first in is a huge advantage.