probably a combination of things. I prefer digital but I acknowledge it’s possible to get excellent results with a good vinyl setup.
Not all lps from back in the day sound great. A whole slew of them sounded much better in their digital incarnation. I know that you listen to a lot of classical . Goddard Lieberson, who headed Columbia classical during the early lp era, wanted his recordings mixed as though the average listener would be listening on AM car radio. Most of the RCAs from seventies had muffled sound and horrible vinyl quality. Play any CD of an Ormandy recording from that era and then compare the lp. I could go on but the point is it depends on what lps you are referencing. In your OP you say that old lps should not sound worse than newer ones, but in many cases precisely the opposite is true.
Worn out stylus? The issues there would be tracking difficulty. However you may want to check the settings on the phono preamp.
And I believe you are in your mid eighties? How preserved is your hearing?