There's something at the heart of them that makes them interesting. It isn't just "exploration", but a particular style of exploration, which will not fail. It isn't just boss fights with dodging, because having that alone doesn't have the heart of a Souls game. The exploration alone could, but yeah, obviously it has to be done properly, not just "exploration".

Realistically, can't both combat and exploration be foundational elements of soulslike?

It could. I honestly don't think it is. Sekiro is also a Souls-like, and the combat is very different. I'm pretty confident you can make a good Souls-like with no combat. The combat, in my opinion, is there to give you a reason to explore. It's there to be a challenge, where exploring is the key to overcoming it. There will be items and secrets that make the combat easier if you search for it. It's not required, but you'd need something else to take the place of this.

Chants of Sennaar comes to mind of something I'd consider to capture a lot of the spirit of Souls games without combat. They're obviously very different games, but I think they share a lot. You can try to brute force your way through, but you're encouraged to explore and engage with the world, and discover it, which makes the critical path easier. This is the goal for both Souls-likes and CoS.