I’m also eager to find out. Depending on whether I keep my carte blanche status or not and the benefits that are associated with it, I will shift my Paris travel to flying instead.
It’s currently roughly the same price for me to fly or take the train (I often book at short notice in standard premier), and it takes me roughly the same time, if not slightly longer for the Eurostar (if there is a queue for the taxis at Gare du Nord), but I chose Eurostar for the convenience of arriving 15-30mins before the train leaves and the frequency of trains.

If they change their program and I have to arrive 45-60mins early, queue, and stand in a crowded departure hall, I’ll simply fly with BA instead.

However I disagree with the fact that everything got worse since SNCF took over. I for once noted some progress recently, the experience is more consistent. At Gare du Nord there is now systematically a Carte Blanche queue/fast-track, when I remember a year or two ago it was very hit-or-miss « sorry sir we are at capacity today just queue with everyone else ». Similarly, the passport checks in the carte blanche queue at St Pancras were often a chokepoint (one custom officer and the machines weren’t working), but that has now been addressed.
I also noted that a barman was back on the 1st floor of the lounge in St Pancras, which I hadnt seen in a while. The food onboard has become (very marginally) better too: it used to be always chicken or vegan quiche, but I was surprised when I was offered another option the other day. I also like their new face-recognition queue.

Fingers crossed!
I’m just annoyed that they don’t communicate anything about the changes. We are their most frequent customers and they are basically rug-pulling us — not good