No credit card required
Browse credit cards from a variety of issuers to see if there's a better card for you.
A few potential benefits to a PC:
1. There's no new account created, so there's no blow to FICO scores from a new account.
2. If you already apply for new accounts and bonuses at a fairly aggressive pace, it can help for 5/24 purposes (and similar other rules with other issuers).
3. Amex HPs less often now on new accounts, but it could potentially save inquiries.
Of course, a new account can mean a bigger bonus sometimes.
The only benefit to doing a PC would be to maintain the account age and prevent any new account dings. With American Express Delta cards, I always strongly suggest going for a new card as there are typically HUGE welcome offers available (i.e. the referral offer right now until 29 JAN is 75k SkyMiles + 10k MQMs), often worth well over $1000. Upgrade bonuses are typically in the 5-10k SkyMiles range.
Even going on the low end of your $6k/mo spend, the Reserve would get you a minimum of 60,000 additional MQMs vs the Platinum's 50,000 additional MQMs with Status Boost. If you do $90k of spend it's 90k MQMs vs 75k MQMs. If you are always well over the MQM marker for Diamond, note that the boost MQMs on Reserve can be gifted to a friend or family member (or even a particularly helpful moderator on a credit forum - LOL joking!). The higher AF is going to give you a companion certificate that can be used for C+ or F, rather than only Main like the Platinum. Using your choice benefits for other items like a $200 gift card or 25k SkyMiles adds additional value (I assume no matter what you are getting RUCs and/or GUCs), since you'd get lounge access with the card the same as now.
Having the Reserve is also helpful for upgrades. Even as a Diamond, flying out of SEA (which I assume from your user ID), I'll assume you're not at 100% upgrades to F/D1? On the upgrade hierarchy, having the Reserve card is the 3rd most important thing only after Medallion status and Fare class. So right now if you and another Diamond both have a K fare in Main Cabin, but he or she has a Reserve card, you've lost your upgrade to them.
Going for the Reserve by 29 JAN will lock you in to the $450 AF for the first year before it increases to $550. The type of card generally makes no difference in starting lines or maximum credit lines. Also keep in mind that you can ask for a CLI up to 3x your current limit on any one card every 181 days with Amex, and with the kind of spend you're putting on the Delta card you should certainly have some room to grow.
Not a problem at all. The Medallion Upgrade Priority list is located here for reference:
https://www.delta.com/us/en/skymiles/medallion-program/medallion-upgrades
Hitting that MQD waiver is also a boost, although you have been doing that by March each year anyway it seems.
While I sincerely appreciate the gesture, referral codes aren't allowed on myFICO. If you have any friends or family members with any Delta card, they can refer you for their card and you can still choose the Reserve 75k RDM and 10k MQM offer. Otherwise, a lucky anonymous person out there has likely put theirs on the web.
You're very welcome. I'll settle for a top-shelf drink in SkyClub the next time I'm at SEA and don't use the Centurion Lounge next door.