You can and should use different nsecs.

But then we need to rework NIP-06 so it generates multiple keys the same way BIPs 44, 49, 84 and 86 generate new keys for each payment.

Too complicated

How do you think people should use different nsecs then? Of course, there can be multiple nsecs for different purposes. But using a different nsec for each post? This improves privacy, and I have made apps myself that do this when not logged in with an existing profile. But without having a seed for this, saving them all manually is nearly impossible.

Different privacy needs, different nsecs. My nsec for health care data for instance is not this one.

So purpose-based, right? I still could use my profile here for sharing things publicly? But... the same way you publish a Lightning address to your kind 0 profile data, why not also put in a Bitcoin address field? This could support single-key addresses (like P2TR, P2PKH etc., still bad for privacy, but not tied to your nsec), but also PM8T (BIP-47) or silent payment addresses. You want ro see these as well, right?

Yep. Most people will have different nsecs for each Nostr app, so in the end, it's all distributed.