Isolationism occurs in any functioning group because people fear losing it, or being drown out by the new users. There's also the small sect of people who seem to have the vocal attitude of "well I figured it out so you shouldn't need my help," which I've run into in varying forms.
I remember it happening on Reddit too. First when the great Digg migration occurred. And at various times later in some subs that shot to frontpage level popularity.
I think we should encourage migration. Lemmy isn't going to shoot to Reddit levels overnight, we're probably seeing a growth that will plateau, then shrink as people miss their niche communities (which we have too few active users to have thrive). If we're very lucky the folks that stick around will grow Lemmy 10ish%. But every time we do that those niche communities become that much more viable and Lemmy in generally becomes more appealing lurkers.
On Lemmy these viral things consume the site for about 3ish days every few months, then fade into lore. The first one I was here for was when were were all obsessed with not pooping for 3 days. There were the beans. There was a longer one where almost every post on the site was some reference to Saddam Hussein's spider hole. There was a period where every upvoted post had to contain one or more black cats.
I've been to the 1st birthday parties of some of my friend's kids. Nothing of value was lost
...that Matthew kid is always a dud.