I agree with you.
On Mastodon, a few weeks ago this topic was raised after the default mobile apps started to streamline signups for the "mastodon.social" server (equivalent to lemmy.ml here). Many were displeased by it, saying that "we cannot have only one big instance, it's Twitter all over again" and something on that line.
But I think it is a good thing, personally, especially since mastodon have an account migration feature. Let the people experience the service, then give them the choices. Other apps and servers use this approach (eg. Mammoth for Mastodon app, Vivaldi browser's instance...)
Lemmy is not as mature right now as Mastodon was during the Twitter migration. This is a challenge, but presents some opportunities for the devs and the community to see what works and what didn't work for all this federation thing. But it does have potential to be "mainstream".
For those who can't access the site, the source file from GitHub: https://github.com/LemmyNet/joinlemmy-site/blob/main/src/assets/news/2023-06-17%20-%20Update%20from%20Lemmy%20after%20the%20Reddit%20blackout.md