The Illegal problem down there is already so bad that hundreds of thousands are pouring across the border now. If you add I-69 that provides the illegals with a direct path to the Heartland.
Why I-69 to Texas???
There are many Interstate routes available to connect Canada to Mexico. I'd extend I-69 down through Evansville, to Memphis, into the Mississippi Delta, through Jackson and to the Mississippi Gulf Coast personally. Most of those areas have no freeways, let alone Interstates.