Couldn't they "convince" instance admins to include ads? Couldn't they flood communities with influencers? Couldn't they promote Lemmy apps that have ads?
You are severely underestimating the criminal energy and creativity of these corporations.
Edit: reddit turned to shit way before the platform itself was up for sale. No need to buy the platform, bots and influencers are enough.
No one needs to buy Lemmy to enshitify it. All of the above and many more methods would result in a shit experience.