It's like Remington Steele vs Moonlighting. People think later seasons of RS weren't as good as the earliest seasons, but they still think it stayed good throughout it's run, with mixed opinions about the 5th season. The series also stopped as soon as Laura and Remington irrevocably got together. Most people think Moonlighting "jumped the shark" when Maddie and David got together. In both cases, the primary motivating factor was the wit (different kinds, but stil...) and the will they/won't they. The dynamic here is similar, but with a less obvious "they will...someday" ultimate outcome.

Whether or not moving things forward with a pairing "spoils" it really depends on the pairings themselves. High School Debut would have gotten old fast if it hadn't gotten Haruna and Yoh fairly quickly, whereas Skip-Beat will probably lose steam shortly after Kyoko and Ren figure things out, if they ever do. Then there's things like Basara, where they're arguably "together" from early in the series, but separated a lot.