I don't think magical footwear needs a nerf, the problem is that other inspiration runes suck.

Magical footwear seems totally balanced to me. You forgo any benefits from that rune for up to 10 minutes, and you even lock yourself out of buying boots. There are so many times where I wished I have around 300 gold but nowhere near enough to buy an amplifying tome that I would have liked to buy boots. It seems perfectly balanced in its current state, and a perfect example of a scaling rune. We need these lategame runes since the removal of "X per lvl" runes (old runes) messed up a lot of old scaling builds. In addition to that, since you can't just grab 4.5% MS at level 1 like you used to, that extra 10 MS is very important for some champions. The other MS runes are simply too conditional, so I feel like waiting 10 mins then saving a bit of gold and gaining 10 MS is entirely fair.

The problem is that other runes in inspiration suck, so we're only really seeing 3 runes being used. **In particular, the other runes in the same column are inferior to perfect timing, magical footwear, and cosmic insight. **

Magical footwear is being taken because minion dematerializer is more trouble than its worth, and future's market is way too niche. In general, most champions would rather buy something else or attempt to stay in lane until they got the gold they needed, rather than go in debt, pay a lending fee, and also miss out on the affects of a different rune.

Perfect timing is being taken because like minion dematerializer, hextech flashtraption is more trouble than its worth, and biscuit delivery offers very similar gold value but simply doesn't compare to an early zhonya's active. Maybe perfect timing needs a nerf even if the other options were better.

Cosmic insight seems to be bothering Riot, usually because inspiration secondaries don't even utilize a rune from the 3rd row, but again the other runes are pretty weak. Approach velocity is niche, and also has tons of overlap. If the enemy is slowed, the extra MS moving towards them is way less important. Celestial body is just flat out counterproductive. 100 health is not even worth 300g, and there are almost no champions that want the health, but also don't care about losing damage to monsters and champions.

In addition to all of this, inspiration runes aside from perfect timing and magical footwear are pretty stupid weak, especially when you consider that inspiration keystones will be very weak after the klepto nerf. If Riot nerfs magical footwear and perfect timing, you won't see players using other inspiration runes, inspiration will just flat out never be used. Supportive champions will opt for resolve and sorcery, or even domination for ghost poros/zombie wards. Also, if phase rush was less awful, it would be another viable option for MS that would compete with magical footwear.

tl;dr the reason you only see the same runes being taken from inspiration is that the other options in the same row are vastly inferior on nearly all builds.