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I've never thought about this. And now I just realized it and started to think about it when I read the topic you made OP. I think this is one of the topics that awakens old insights that must be brought up again by the seniors here who know about this. Thank you friends for creating a topic which I think is a good question that really represents the curiosity of every newbie here, including me personally who just found out about this.
Blockchain.com wasn't the one who built the first blockexplorer. By the way, they were known as Blockchain.info prior to that. You are right; no one actually needed to see the balance of someone else's address and most of the information that you see on the blockexplorers are not essential to most.
The first blockexplorer was actually created by theymos:
https://bitcointalk.org/index.php?topic=1727.0. This was before blockexplorer.com and it led to the creation of that which was a lot more polished and with more detailed information.
This is a new insight for me. Thank you guys for sharing.