Is it just my narrow selection of experience, or do all long-running sci-fi series eventually devolve to focus on ordinary interpersonal drama over focusing on exploring speculative scenarios?

It sure happens a lot. I got tired of the Expanse when it turned into just being geopolitics in space. Still good in it's own way, just not what I'm there for.

I guess Revelation Space stays speculative. It just kinda gets worse after a while. It doesn't have a lot of character continuity between books, which might help with leaving the stage open for wacky scifi concepts. (A lot of the books are really good, but Redemption Ark was meh and I found Absolution Gap disappointing.)