This is a surprising opinion to me! Subjectively, I very strongly feel that models improved significantly over the last two years, particularly around the time

This is a surprising opinion to me! Subjectively, I very strongly feel that models improved significantly over the last two years, particularly around the time of Opus 4.6 and now Sol 5.6 and such. It's made a huge difference to how much autonomy I can give them. If you trust benchmarks (which is debateable) then the rate of improvement is speeding up -- definitely not diminishing returns.