@NexusD
Sansui isn't a "lowly servant"; he's the equivalent of the duke's champion knight.
Remember: Each of the 4 duke houses have power on par with (if not greater than) the Royal Family itself.
In other words, Sansui is the equivalent of a royal champion or personal royal knight. He's the frickin bodyguard of one of the most powerful people in the entire kingdom.
Pledging himself to the duke means that he and his daughter are protected vassals of said duke house. That means they are shielded from any political threats by the political power of the dukes (which is the reason why the Royals couldn't take Rain from him, nevermind his abilities).
Additionally, it means that Rain has a secured, wealthy upbringing from someone who ISN'T a 500-years-out-of-date swordsman with no money. The duke house is paying for her food and highclass tuition, and they're presumably also providing her with security.
Sure, Sansui probably won't lose to anyone in a straight up fight. But, a mysterious country bumpkin swordsman with no money, no background, and absolutely no idea about the "modern" society would potentially have a very hard time trying to raise a child.
Sansui might be practically immortal, but Rain IS NOT.
Sansui's greatest asset worth offering is his own capabilities as a swordsman, so he took advantage of that and secured a ridiculously good deal.
He's not some super mage or isekai "genius" (that plagiarizes modern tech and hybridizes it with magic or some bs).
He's not cliche Japanese isekai protagonist with god-given cheat powers that gets a harem even though he barely deserves it (*coughSaigacough*)
He's a 500 year old hermit swordsman isekai normie Japanese that knows how to swing a sword (and some other hermit stuff) REALLY good.
Heck, the English translation of the title is literally "Even if the Sword Saint is Boring He's Still the Strongest".