TL;DR: Integrity, or instance identity, and outreach are both important to help federated online spaces to continue to exist.
This is meant as a kind of complementary piece to sabreW4K3's post, Why Integrity Matters (link to thread on their home instance).
I'm writing this as I don't entirely agree with, nor disagree with them, and want to provide another perspective.
Integrity, or as I see it, instance identity, does matter insofar as one wants to build a distinct community that anyone cares about. At the same time, online communities typically aren't self-sustaining in the same ways offline communities can be. Online communities benefit from both integrity and outreach to sustain themselves.
Lemm.ee going offline soon is as much an indicator of this as anything. Calls for additional admins to help offset burnout went unanswered, and while there are many reasons for this, one among those may be as simple as insufficient interest or care for the instance. With Lemm.ee being a "General-purpose" instance, it never developed much of an identity, nor a local community with much attachment or interest in its maintenance.
At the same time, it's unclear how much more outreach was attempted to get more help with its administration, and beyond that, to draw in more people that might care enough to build a distinct community on Lemm.ee to in turn find those interested enough to join its admins.
Simply put, more people helps to delegate the responsibilities of moderation and reduces some of the burden of admins. It can also help indirectly to "moderate" the feeling/atmosphere of an instance to have more people with more varied interests participating.
Without integrity, each instance is at risk of disinterest in it remaining, and without outreach, each instance is at risk of growing stagnant. In the absolute worst case, without both you not only risk losing instances, or communities, but possibly development of the software (whether it's Lemmy, Mbin, or Piefed or so on) itself in the long term.
Each instance having its own identity is one of the key advantages of federated platforms like this. (Broadly speaking, it's true of decentralised movements and organisations too; each group can have its own unique identity and focus without compromising on what all such groups have in common.) Finding people who appreciate the focus of a particular instance enough to choose it over another may be the challenge, but as you wrote, it's important in order to avoid stagnation and to be able to keep the instance going. It's possible that the unique aspects of an instance may be what draws in certain people who otherwise wouldn't participate at all, making instance integrity/identity beneficial to outreach in such cases, not at odds with it. As we saw with lemm.ee, appealing to a general audience doesn't tend to attract the "extremists" with motivation to keep things going.