She built well into Lethality even before patch 7.2, which buffed Lethality even more, pushing her even further up. These changes were a long time coming. I'm guessing you've obviously never had an MF instantly proc Black Cleaver on ult and shred through even your tanks' armor. Black Cleaver recipe changes hardly stopped her from building it, either. This patch doesn't do much to lessen that specific pain point either.
They're not trying to gut her. They're trying to convert her back to crit because Lethality MF is hella unhealthy.
On the subject of
Varus, his nerfs were very much needed, too. Because it's not a very interactive play pattern. It's essentially free harass and waveclear all packed into one. Riot wants to make lanes more "interactive". Where there's never 'nothing you can do' about a match up. Its why they nerfed
Zyra the way they did. I'm a Zyra main and I actually admire that way they nerfed Zyra. It trimmed her free power WITHOUT punishing those who put time and effort into learning the intricacies of her kit. Why punish mastery? Especially on a champ like Zyra who can be relatively tough to master. Instead of just putting plants up on a whim for free zoning, Zyra has to get closer into the fray and risk more (she's the epitome of squish) to actually damage you with her spells... though Zyras that are actually good already did that so it doesn't hurt them quite as much.
Now compare that to Varus's situation...
#... Q-SPAM MECHANICAL OUTPLAYS!
They want to have him get closer for his damage and enforce the Blight detonation gameplay, which makes his lane more interactive since he has to get closer to apply stacks... where you can interact more and punish him for mistakes.
The dudes in charge of these nerfs are geniuses IMO.