"maybe these old bulls have no merit, even on traits we can't see or measure yet, maybe they do"
they do.
We used a full sib to ABR Sir Arnold G809 (G809 semen was no longer on the market) and a couple of G809 sons, back in the 1980s, and more or less linebred, in-herd, to him for years.
He was a 5-trait leader (CE, BW, YW, WW, CEM - if I recall correctly), and those Fleckvieh-influenced SimAngus cows were GOOD! Had several that were closely linebred PollFleck 809 X Alpine Polled Proto that lasted to 18 years of age, and were pretty moderate frame for cows born in 1990.
I'd use G809 again, in a heartbeat, over Angus or SimAngus cows. We did use a fullblood Fleckvieh sire that traced back to G809, top and bottom of his pedigree, over select 3/4AN-1/4SM cows the last two years.