It's always hard to compare athletes of yesteryear to those today. Old timers like me may be inclined to think Mario, A.J., Sneva, etc., are unequaled, but I think today's talent is a good as ever, and right now there's a great mix of aging and very young talent, which is fun.
More than ever before Indy racing pulls from a global talent pool, and that in itself argues for the strength of today's drivers.
On the other hand, increased costs have put a premium on drivers who can bring in their own money, instead of being hired solely for their skill. So there may be doubts about the bottom level drivers. Yet in the older, less expensive, days, drivers could build their own cars, and some may have made it more on mechanical skill than driving skill.
One difference is that drivers today are far more fit. Any old timer who had the grit for extreme fitness could compete today, but a great driver back then without that willingness would get left behind.
The most insurmountable difference for some would be size. Drivers tend to be smaller today because weight matters so much. Some of the big old guys, whose size helped them grapple with cars without power steering, would be hopelessly handicapped by their size today. They'd be no less skilled overall than today's drivers, but their weight would cost them precious hundredths of a second per lap.