It's not zero, each fiefdom has very little power to keep users. As it is right now, a user unhappy with their instance culture or laws can move to another instance. Comparing it to moving in real life, in real life you have a lifetime worth of things that tie you to your fiefdom. Comparing it to well established and centralised social media, then those fiefdoms still have a lot of power over you.
Your social network can't come with you, they're SSO providers, they're tracking and human-verification providers, they have high quality exclusive content, they're sometimes the only channel for interacting with some third parties you have to interact with (Government, utility company, etc).
Instances are worthless, what has value are the /c/ and absolutely nothing in the Lemmy model protects communities from the admin of the instance where it was created to go full Elon. I bet that at some point it will happen.
Most of the time you don't even know who is running the instance. Suffice that one of them that's running a large enough communities needs a bit of cash and decide to sell it. Or they could be in bed/owned by any intelligence agency/corporation/political party. Who knows.
I've spend a year in my lost time musing on the design of a truly decentralised model where identity, community, curation (moderation) and distribution are entirely decorrelated to address those specific issue among all the othes, including the one you mentioned. It's complex, it's a big task, but I don't think it's impossible. I'm too lazy to code it though :D
absolutely nothing in the Lemmy model protects communities from the admin of the instance where it was created to go full Elon
I'd say the low cost of migration does, especially if user awareness remains high (and since most users are here over complaints of the APIs being restricted, I'd say there's an above-average awareness). It's pretty easy to clone a community onto another instance, and it would be trivial for users to migrate too.
As you discovered when you tried to get your friends to use Signal instead of whatsapp it's actually very hard to move people.
Everyone was "yeah let's leave Reddit the owner are evil and taking away our mobile apps". Barely anyone did. It is not trivial to convince a group of people to move.
My point is that it's very different from moving from WhatsApp to Signal, or from reddit to Lemmy.
Let's imagine on an instance, a community mod started flooding their /c/technology with ads and deleting any posts criticising them. And suppose the admins decide not to step in, saying it's their community and their right to do that.
How painful would it be for users to go from /c/technology over to /c/tech or /c/technology@other.site ? There is a far smaller barrier - it's basically two clicks on their side to change their comm subscriptions, they don't risk losing communication with friends or miss out on a larger site's content feeds, or have to deal with 'one more app', they don't have to learn a new tool, they just use a different community.