Your copy of the blockchain is YOUR copy. It doesn't do anyone else a bit of good whatsoever except insofar as there is additional redundancy, but that's a diminishing benefit.

An example might be in order. Say there was only one copy of Shakespeare's complete works in existence. The first duplicate creating some minimal amount of redundancy woult be incredibly valuable to have, so long as it was kept in a separate location. A third duplicate would make it even less likely that they would ever be destroyed, but it wouldn't be as valuable to society as that second copy was. By the time there are tens of thousands of copies of Shakespeare's complete works, the chance that all copies will be destroyed and forever lost are virtually zero, so the next copy that is added, while it technically helps ensure this is the case, it's really not measurably meaningful for anyone other than the person who owns that copy, because there may be tens of thousands like it, but that copy is THEIRS.

Same thing with Bitcoin nodes. Running a node does mean it is marginally harder for Bitcoin to ever be destroyed, but it's not like Bitcoin is meaningfully less resilient without your node.

Your node does relay pending transactions and newly found blocks to its peers, but they don't rely on your node alone for that information either. They would have just received it from a different peer instead.

Now, that doesn't mean running a node doesn't matter. It most certainly DOES! It's just that it primarily matters for YOU, and not anyone else. So long as there is reason for people to run a node for their own use, and it remains relatively easy to do so, node-count will continue to grow and Bitcoin will remain resilient.

If no one has any real reason to run a node for their own use, though, node-count will start to dwindle and Bitcoin will become less and less resilient, one node shutting down at a time.