Umm half of those aren't counters to slows. Talisman isn't a counter to a slow, Boots of Swiftness isn't a counter to a slow, Ghost isn't a counter to a slow. Notice how most counters to a slow (dashes/blink) are on cooldowns that aren't 200 seconds. If Ashe can slow you every autoattack, if any mage can build a rylai's, if any ADC can build a frozen mallet, then it's not fair that nobody should be able to catch up to them ever.
You also have abilities like Leona's E which is a skillshot dash and wouldn't make sense any other way. In fact she'd probably be ridiculously overpowered if she got a huge MS boost so that nobody could try to sidestep her stun.
Counters for dashes already exist in game too so it's not like they're OP. You got Snares, Roots, Stuns, Silences. You also have in game walls like Azir's ult that don't allow most dashes through.