The Platinum charge card definitely doesn't cancel out the SkyMiles Platinum card, and they can certainly complement each other. Many Amex products with annual fees are going to have overlapping benefits, so it's really a matter of evaluating the perks and rewards earnings to ensure that they outweigh the annual fee.
As you already have the SkyMiles Platinum card, there are some additional possibilities with adding the Platinum charge card. The ability to earn 5X on flights booked either through Amex Travel or directly with airlines is a huge one for a frequent flyer, even if they are on Delta. To just compare apples to apples, you earn 3x SkyMiles on direct Delta purchases with the Delta card, but you would earn 5x MRs which can be directly converted to 5x SkyMiles or have many other flexible uses with the Platinum card. If you book hotels through Amex Travel, you would earn 5x MRs (which again can be flexible or turned directly into 5x SkyMiles) rather than the 3x on the Delta card - however, it's important to note that if you have/want hotel status (including even the Gold status with Hilton and Marriott that comes with the Platinum card) you will likely want to book directly with the hotel chain as elite benefits and earnings generally won't happen when booking through an OTA like Amex Travel; in this case earning the 3x SkyMiles on the Delta card is going to almost always be a better option than earning 1x MRs on Amex Platinum. If you don't have a card with higher earnings for restaurants and groceries, the SkyMiles Platinum card will also typically be a better earner with its 2x SkyMiles vs 1 MR.
As mentioned, access to The Centurion Lounge, lounges in other networks (some exclusive to Amex, most with Priority Pass), and Delta SkyClub (when flying Delta) that come with the Platinum card can be huge for a frequent traveler. IF you use enough of the monthly/semi-annual/annual credits, the net annual fee which does look steep at first glance can land somewhere between minimal and net negative (supposing you actually used all of them, that's around $2000 a year - but it's unlikely that many customers would organically hit all of the (limited) digital entertainment, Walmart, Sak's, Equinox, SoulCycle, and FHR credits in addition to the more general uses of Uber, airline credit, PreCheck/Global Entry, and Clear that would be fairly simple for many frequent travelers.
For a loyal Delta flyer, the more likely scenario of a Delta card and the Platinum charge card overlapping too much would be when comparing it to Delta Reserve rather than Delta Platinum, as that also grants SkyClub and Centurion Lounge access (when flying Delta), one of the key reasons to hold the Reserve card. The free first checked bag for you and up to 8 companions, Status Boost (at $25k), MQD Waiver (at $25k or $250k), and the companion certificate can generally pay for the Delta Platinum card and then some. However, a step up to the Reserve card may make more sense if you would normally be buying tickets in First (or at times with pricing even Comfort+), if the higher MQM bonus would make sense at its $30k rather than $25k marks, and if you might benefit from the upgrade tie-breaker (holding the Reserve card puts a Delta flyer higher on the upgrade list than someone at the same elite status level and fare class) or even possible upgrades processed after Silver Medallions.
Thanks, this really helps a lot!
