If someone never texts first, but always responds, should you keep talking to them?

Probably not. At least not in my experience. They may not like you but are too polite to say so.

I only reply if someone messages me or replies to my messages.
Otherwise I figure they're tired of talking to me.

My honest advice, find out what MBTI personality type they are. It explains so much about people's intentions

For example extroverted people will reach out to you if they sense you are stressed, wanting to change your mood. Introverted people will give you space, waiting for you to seem ready to talk. Knowing which type you're dealing with gives a lot of context to their decisions

I used to do that with a friend.. I didn’t especially like talking to them but was in the habit of replying

Do you like talking to the person? Can you ask the person why they don't text first? Does the person possibly have low self-confidence and doesn't feel as though they have anything to say?

I'm the person who doesn't message first - usually because of some combination of not wanting to be a bother, having nothing to say at the moment, and crippling stress/anxiety.

I probably wouldn’t bother.

Yes of course
That doesn’t mean they would make a great gf or bf
Who is first is a little silly
I’m always first to write to my favorite people and they LIvE with me!

I’d feel like a bother

You stop texting first.

I'd say definitely not