There's no ownership behind hashtags. Can't really build on them much.
Npus offer waaay more options. (like building a community around them, for people with embarrassing questions, for example 😉). It also makes it a lot clearer what your target audience is for your question.
I think there's a lot to consider with having a single key
1. I would own the key and would become responsible for the content, which would lead to eventual censorship
2. I have the maintain the private key secret, which is a security concern from a development perspective
3. If a single public key gains a large following then users could take advantage to advertise, promote, spam or outright infect the feeds of those who were generous enough to follow
I'm reluctant to go down this path
