The Yuumi W changes provide more opportunity for Yuumi to be caught outside of an ally champion. Yuumi _can_ buy boots but, most Yuumi players have avoided doing so since they didn't need it. No boots in turn has meant a Yuumi can't escape alone. No boots has has also meant that Yuumi has an awkward time going from base to lane without an ally to attach to.
On paper the changes somewhat encourage boots on Yuumi, but not enough that it's a clearly good decision. I'd assume it'll still feel very wasteful to buy them on Yuumi.
I want to suggest a simple alteration to the new W for consideration. **Have the 0.25s channel time scale with level of boots.** Tier 2 boots? Faster attaching. Do so and nobody would question a Yuumi in a boot.