By having more instances and better user distribution. Running a small-ish instance isn't very expensive, around 5-10 euro a month (some VPS providers are cheaper, etc). As Lemmy development continues, and more optimizations come in, these smaller lemmy instances will be able to support more users.
There is also a discussion on GitHub to introduce user and community migrations between instances. So once that feature is implemented, it will be easier to redistribute everything across all Lemmy instances.