Chase implemented the “one Sapphire rule” suddenly, without notice, as
@gottalovepluto noted, at the end of August 2017. The concept of the “double dip” had been around for much longer before the one Sapphire rule took effect, but it had been used primarily when someone was at 4/24 to “double dip” two cards with that final slot and end up at 6/24. Otherwise, if they had gone for just one card with their final slot, they would have likely needed to wait months for their next oldest card to drop off 5/24. When the one Sapphire rule hit, people immediately applied the double dip as a workaround to the new restriction, because the mechanics are the same. You want to get approved for the second card before the system updates to recognize you’ve already been approved for a first card that should be preclusive.
If the loophole were to close, you’re right that it’s not the end of the world. It’s 50k UR you’ll be missing out on, but those points would have come at a cost of one under 5/24 slot. And that would free you to get other cards with comparable value.
Previously, the popular thought was to eat the $95 AF for the big bonus, then cancel or product change the CIP (to a CIC) after a year. But, the CIP’s 3x earning ability on Plastiq, Gyft, Swych, Venmo, eBay fees and shipping, etc., the 1.25x redemption bonus if you don’t have the CSP or CSR, and ability to generate referral bonuses (the CIC and CIU don’t have referrals) has made it a keeper card for a lot of people. In addition, with the CIC and CIU each having 50k UR signup bonuses, the shift in thinking is to hold on to the CIP if the numbers work out, cancel the CIP if not, and apply for the CIC and/or CIU separately so you can get the signup bonus. You don’t get a bonus for product changing your CIP to the CIC/CIU.
3 business card on your one SSN (without resorting to signing up for EINs), with applications sufficiently spaced out, and keeping overall credit lines low to increase your chances of auto-approval, seems to be about the upper limit of what Chase will approve.