I wish that Lemmy is the only social network from now on.

Granted. Everyone from twitter, facebook, reddit and tumblr now hang out here exclusively. Lemmy adapts to the new audience over time. Some of the larger instances add monetisation and advertising in order to offset costs, and then they fall to greed. Corporations try to buy instances to turn into business ventures and dickwads try to turn smaller instances into investments. Lemmy becomes exactly what it was created to avoid.

The monkey's paw part is that since you specified actual Lemmy, that means the software is still controlled by the original devs, and moderation tools continue to develop at the same pace.
It gets flooded by influencers, scammers, viruses, and corporations.
Another interesting side effect is that since that means the entire population of China -17% of world population - switches from Weibo and Weichat (not to mention Douban etc) then Lemmy becomes a major focus of both CCP propaganda/firewalls and hackers.
The dark net is now carried out on Lemmy too. So is your friend's racist grandma, all Putin's trolls, and millions of Hindu nationalists, the contents of Truth Social, etc etc.

Granted. Because you didn't specify online social network, all human interaction now can only be done through Lemmy.
People can't make phone calls. They can't talk to each other face-to-face. If you're standing in line at the airport, security is done exclusively through automated systems. If you need tech support, it must be done through Lemmy.
The federated system cannot support that must traffic efficiently. So Lemmy periodically defederates, meaning your Taco Bell order can take six weeks or more to go through. Surgeries are now impossible, because doctors are not able to directly talk to support staff. Human population plummets.
Eventually, all electronic systems start to break down. Lemmy begins to fail, and all of humanity is doomed to individual isolation.
- And that's a Tuesday!