Movement soft caps. At 415 MS, you get 80% effect from further increases to MS. At 490 MS, you get 50% effect from further increases to MS (so when your MS is worth 508.75, your MS is 490 and you hit the seconds MS cap, halving all increases in MS after that).
And yet, people can regularly break 1000 MS on champs with MS-based mobility.
Just to clarify, sight range is 1200, so moving at 1000 means that you're almost reaching the fog of war within 1 second.
There's a very good reason why people will always take Homeguard over Distortion with Teleport.
Anyway, on the subject of MS versus Dashes/Blinks, it's dependent on the situation and the specific dash. How far does the dash travel? What is its cooldown? Is it free-roam or targeted?
There are plenty of situations where having literally 5 more MS than your opponent helps you survive, just as often as there are situations where moving double their speed won't save you. One is not expressly, unambiguously better than the other.