I'm not saying there is not anything behind shin kicks but it would seem weird to think that all the karate, kung-fu etc. martial arts going back centuries if not millenia would be wrong in their ways to advocate mostly foot strikes. As I stated, striking with the foot has the advantage of greater reach and making yourself less vulnerable to counter-strikes and takedowns which are more of a risk when striking with the leg as opposed to the foot.
In addition, a lot of kicks are plain impossible as anything but foot kicks, back kicks being the most obvious example. People may hurt their feet but they are generally protected by footwear and if a person was called upon to kick a hard object such as a tree, I'm guessing 90% or more would opt to hit it with their foot rather than their shin, as the latter would cause much greater pain and injury. Just like if you were asked to jump on top of a beam, you would prefer to land on your feet rather than your shins as it would result in agony. As I said, the feet are naturally built to deal with this kind of force, as required in running or jumping. The force is transmitted harmoniously through the foot and up the leg. It seems pretty natural. In the opposite case, a splicing kind of force is delivered to the leg as when you're breaking a board in half, only your leg is the board. Soccer players and many other sportsmen wear shin protection as well, but no special foot protection.
Feet are more specific and the force delivered by the foot is spread more specifically across a smaller target area, making it more dangerous in that sense. It's like comparing a scattered shotgun blast to a pistol. That's why a fist in the temple can be lethal, but a forearm would not be that scary unless it ended up cranking the person's neck.
So I'm saying that both have their place and shins are not superior except in the instances where they are, like when you're hopefully not a skinny person running it to somebody else's far sturdier leg. As for your straight line thing; roundhouses are not straight. They are like the lower limb equivalents of hooks in boxing and by that logic, somewhat slower. And unless you're slamming your shin in your opponent's groin (which does not seem to be advocated by many martial arts) then you're not kicking in a straight line. You need to use your feet (or knees at close range) for that. I hope nobody gives me trouble over calling those thai kicks (as in the above videos) roundhouses just because you may not call them that. It's just that it would require a very mutilated, mutant kind of anatomy of one to hit someone in the side like that, with your shin, in a straight fashion.
As for thai tradition, there are chinese styles (for example) that swear by never raising your feet far off the ground. If the general development of thai/southeastern society is of any hint regarding the solidity of their fighting science, I would much sooner side with the northeastern sphere of influence.