"A Boy Named Sue" and "Joshua" both contain what we now call "rap," so they would sound alike in that regard, but ONLY in that regard. (A comedian friend of mine calls "A Boy Named Sue" the greatest country rap song of all-time, with "Convoy" in second place.) I've had both songs for 40 years and I cannot hear any melodic similarities.
And yes, the modern stuff does sound alike because "programming consultants" have conducted research and discovered that people don't want variety, they want 80 clones of what they like. While this is fairly new to country music, it's hardly new in music: listen to any disco song, for example, or notice how rock bands like Boston and Journey were so formulaic that even their ALBUM COVERS were identical. It was bound to make it to country sooner or later. You can find the advent of this in the music of Alabama. (If you wanted to claim "sound-alike," a better option would have been to compare "Mountain Music" and "Tennessee River" by Alabama, which are so identical that the fiddle break could be removed from one song and inserted in the other without anyone noticing it!)
It is not universally the case in country, as it is not in rock. You have to remember that recorded country music is NINETY YEARS OLD, so you can definitely find similarities ("I'm Thinking Tonight of My Blue Eyes," "Great Speckle Bird," "Wild Side of Life," and "It Wasn't God Who Made Honky Tonk Angels" all use the same melody). However, just because three or four -- or even a hundred -- songs sound similar, it is a drop in the bucket compared to the literal MILLIONS of songs that have been recorded over the past nine decades. Listen to "I'm So Lonesome I Could Cry," "Whose Shoulder Will You Cry On," "Highway 40 Blues," "I'm No Stranger to the Rain," and "My Blue Tears" and tell me where the similarities are.