Mexican military expenditure is a very small proportion of GDP, lower than almost any other country, & very little of it is spent on things which would be saved by the abolition of the army.
The navy functions mostly as a coast guard, doing fisheries protection, chasing smugglers, guarding oil rigs & the like. Those functions, & the equipment for them, will still be needed.
The air force has a purely token fighter force, enough only for air policing of the region around the capital. Probably worth keeping - indeed, it is arguable that it should have more, if only to be able to deal with smugglers & the like. Its reconnaissance aircraft are necessary if Mexican air space is not to be free for the passage of any villain who wishes to transport drugs, or whatever.
Air force transport is used to support the army, & for VIP transport. There isn't much of either, for a country as large & prosperous (on a world scales) as Mexico. What's to abolish?
The army - well, now, what does it do? Internal security & border patrol. As you say, a "National Guard" can do that - but what difference would it make if it changed its name? It is mainly equipped like your hypothetical "National Guard", with light AFVs & infantry weapons. You can argue that it should get rid of its small numbers of anti-tank weapons, its very old tracked AFVs, & its old light artillery - but frankly, they don't cost much to keep. As long as it doesn't buy lots more, they're not wasting much money.
Whether Mexico needs as many soldiers as it has is debatable - but its army is small in relation to population. Some of its weapons are pointless for national defence - but only a few. Converting it into a para-military pure internal security force would change very little except the name, & save very little money.