Do skyshards respawn?
Every level have two parts:
1) stats
2) skill lines
Max stats end at max level of 50 (beside adding a little more via enchantments)
They are so many many skill lines and more are being plan to be added during patches. With every 3 skyshard, one get one more skill point to spend on a skill line.
1) If skyshard do NOT respawn, then there is a finite points to be had, thus one can not master all skill lines.
or
2) If skyshard do respawn and in theory with enough time, exploring and luck, in theory, one can get enough points to fill all (hundreds) of points in a skill line.
Also, one can not spend a point on a "bad skill" in a skill line since one can get an other one later to make up for that "bad mistake in the build" It kinda makes respec a moot point. Makes in stats are NOT so forgiving in this way.
So, do they respawn?
Respawn means that one can get the same skyshard later after a certain amount of time has pass since last time one get that skyshard.