Agreed. And limiting access on the vanilla Amex Plat while maintaining unlimited access on the DL Reserve Card would be totally out of Amex's playbook-- some people (note that I'm saying *some*) who only hold the Plat might then add the Reserve as well, increasing Amex's annual fee revenue.
Thinking of this year's changes to Centurion Lounge guest access, I believe overcrowding was only one consideration for Amex-- the other being increased guest fee revenue and increased annual fee revenue from people adding AUs (or increased interchange from people who are compelled to spend $75k to unlock guest access).
As a vanilla Plat holder, I'm thinking this would result in me booking fewer DL flights than I do today. All else being equal domestically (a few times a year), I prefer DL where this a SkyClub and have no issues paying for a guest. If they're going to lock me out after a few trips, continue to charge for a guest (I'm assuming guests can't use any of the allotted visits) the tiebreaker no longer goes to Delta.