For what it's worth, I got into the series a bit late myself and didn't play AC1 until probably 2012 or 2013. I asked people at the time whether it was worth it to play AC1 or if I should just skip ahead, and everyone told me to skip ahead for a lot of the same reasons you said.

I'm not sure why I bothered asking because I decided to just play AC1 first anyways and still managed to enjoy it.

If you're new to the series, it's still fun just to run around and climb things and kill people. The story was actually interesting and the Animus was a really cool concept. The occasional shift to present day gameplay helped keep the historical stuff fresh. It doesn't have the overwhelming volume of useless collectibles strewn about everywhere like later games have.

I haven't played it since so maybe it's aged worse than I remember. It doesn't have dozens to hundreds of hours of gameplay and side quests in a huge open world. But for a dozen hours in a game focused on a linear main story it was pretty good. Like if you took down the walls of the hallways in Uncharted.