This is a good idea I theory so that these small communities gain more activity by being merged into a larger one. But the question is how to actually do it in practice. Many of these small communities are essentially abandoned, with mods that havent posted in months (or never posted at all). The only option is then to have admins of each community instance help with the migration.
Mastodon seems like a better comparison. It has more than a dozen forks and clones, and plenty of donation income.
Sure it would be good to have more contributions in Lemmy, but as these projects are made by volunteers they will do what they are most interested in. Nothing we can do to change that. And if they add new features which prove useful, they can also be added to Lemmy.
New users for Piefed and Sublinks are most likely to come out of the millions of Reddit users, not out of a few thousand Lemmy users. So this will increase the size of the Lemmy network and lead to more activity.
Having other projects which are similar to Lemmy is a great sign. It means users have more choices available and developers can experiment with different solutions. It’s really not a competition, because the existence of more compatible Fediverse projects will also benefit Lemmy, as there will be more users and more content.