Curious.
Instinctively, I tend to factor in a certains bias for "kids these days." Seems a lot of us have a tendency to think educational standards and/or kids' level of knowledge is poorer than before. That said, in the 80s/90s I had a lot of Russian school friends in my neighborhood. Even for primary school age, they had special "russian math" classes taught by a neighborhood parent in the afternoons. They basically followed the old 70s-80s curriculum that you guys are talking about. So basically, I'm convinced it's not just the codger instinct. They were 3-4 years ahead of the 3rd & 4th graders.
These days, I have a few eastern european (polish, slovak & lithuanian) friends with schoolkids of their own. They had the same view of the modern/western curriculum as my childhood friends' parents had. It's substandard. Two years ago one couple moved back to lithuania. She reckons the modern standard for math is as bad as it is here, these days.
To hazard a guess, I think that the old/soviet-ish system was very rigorous but not very enjoyable for the kids, especially below average kids. For example, it was very sequential and intolerant of a bad week. But, it probably produced a much better educated top half. Also, I think the old educational systems of the later soviet era got a lot of criticism for being too focused on math & science, with not enough creativity, humanities and such. Soviet era "liberal arts" were ideologically impacted, and I think people developed a tendency to just stay away.
But much of Eastern Europe has a strong tradition of arts and humanities, and a cultural tendency to promote it. From a purely traditional-cultural perspective, many Lithuanians would like as much art and philosophy in their kids education as French parents, the European high watermark for teenage philosophy classes. I think there was pressure to get these subjects into the system, with due respect.
Anyway, there's plenty of proofs-of-concept that math & science education could be a lot better. We're nowhere near the ideal pedagogical system.