Who are they defending against? Paraguay? Uruguay?
The main argument used to justify such purchases in regions of low military tensions is to "maintain operational doctrine". It can take decades for a country to properly learn how to deploy and use sophisticated and complex weapons systems, such as modern tanks, high performance aircraft, missile frigates, submarines, etc...
If at some point your country no longer have those system, that knowledge base and doctrine will eventually wither and die. So if the geo-political circumstances in the region change, it will take decades for that knowledge set to be rebuild again, as it will need to be rebuild from scratch, and the country might not be able to afford the wait.
40 years ago Argentina had the most capable and advanced air force in Latin America and arguably the most powerful air force in the entire non-aligned world. All that knowhow is being lost and it will be entirely gone soon if something is not done to give the air force some access to modern military hardware.
By the way, I am not defending the argument, I am just explaining what the argument is.