Benefits have indeed gotten worse on average in my experience, and here are two hard examples. Leave and healthcare.
Leave: It used to be that most companies had a formula for accruing leave, and that vacation and sick were separate buckets and you could accumulate quite a bit. You earn X hours of vacation and Y hours of sick leave per 40 hours worked. This was nice because it was an asset that belonged to the employee and had to be paid out upon separation. Most companies also paid out unused leave over the rollover amount at each year end, and working overtime meant more leave would accrue. Now most companies put everything in the same bucket, give you 30 days, and it's "use it or lose it." This amounts to financial engineering on the company's part, as it makes their balance sheet look better by not carrying all those liabilities, and they generally twist it to make it seem better for the employee.
Health insurance: this just adds to the cacophony of voices lamenting the state of health care in the US today. When I first entered the workforce in 2002 health insurance provided way better benefits, more doctors took the typical insurance plans provided, you didn't have to fight the insurance company, file pre-authorization paperwork, etc. Now every time I get a medical procedure it's a battle waged via paperwork.