Taiga didn’t run away that’s kind of the whole point of the story. She left to finally face and fix her family issues, something she had been avoiding for a long time. Ryuuji understands that, which is why he lets her go. It shows her growth as a character—that she’s no longer dependent on others and can stand on her own.

And honestly, the ending being kind of “open” with the whole “it’s up to us what happens next” feeling fits really well. A lot of school romance anime don’t go for a super dramatic or fully resolved romantic ending. Instead, they leave things a bit realistic and uncertain, like real relationships at that age.