IIRC when Eurostar started it had Standard and First.
They then brought in a Premier First which was the fully flexible tickets, which (for a time) could be interchanged with British Midland business class flights from either CDG or BRU to/from London. Free taxis in London rings a bell.
They then updated it to Leisure First / Business First. Leisure First was great, essentially what the Business Premier is now food-wise, with a choice of hot meals and wines and free-flowing champagne (a *lot* was distributed on Friday evening trains IIRC). LF didn't come with lounge access and was very keenly priced (I think I paid £140 return for a Paris weekend in 2007 just before STP opened)
Then they introduced the current Standard Premier and Business Premier model around 2010?
When SP was introduced the food was almost universally panned - they went from hot meals to a tray of three tiny plates of canapes pretty much, I remember a single small roll with a slice of cheese, chopped carrots with one prawn, and a couple of tiny slices of saucisson. Breakfast was a yoghurt and roll and pastry. As a result SP food was slightly improved to the get to the cold-salad style main course + dessert which you have today.
My thoughts on SP vs Standard: If I am on my own, I will always aim for SP for the reasons Bletchleyite states. For a couple, or 3/4 people, it's Standard all the way especially if you can sit around a table. The food in SP is nice but I tend to self-provision in Standard as you can take bottles and corkscrews through security with no issues.
I would actually compare the seating with the rival operators on the route i.e. the airlines, where Eurostar Standard is certainly more comfortable and with more legroom than an economy airline seat.