If you use pay with miles for DL tickets, you get approximately (due to some rounding) one cent per mile, so the 70,000 DL RDMs are worth at least about $700, which is less than the annual fee. You get a couple extra perks from the DL Reserve card that aren't on the regular AmEx Plat charge card, such as 15,000 RDMs and 15,000 MQMs (which probably won't benefit the OP but can be gifted) at $30,000 spend, two extra RDMs for every dollar spend on DL tickets charged to a DL AmEx card, and discounts for onboard F&B purchases. It's also the first tiebreaker for (free elite) upgrades after elite status tier and fare class, so it should help you to get a couple more upgrades if you tend to be on the bubble.

To me, this says that the Reserve card is worth the fee for at least the first year, and it can be priceless if you (or someone you know) needs the MQMs for status.