Good post - I thought similar, however I couldn't recall what the size of the grounds were at that stage.
OK. I'll deal with 1986, because I can't be bothered to do it all.
There were just over 100,000 at the three tests. Very impressive, indeed.
However, the Old Trafford test had just over 50,000 people in attendance. The capacity of Old Trafford at the time was over 56,000.
The Elland Road test had a gate of just over 30,000. Dreadful ground though it was, it still held over 40,000 at that time.
The Central Park test had just over 20,000. The capacity at the time was about 35,000 (there were over that for the WCC game the following year).
So I make it that there were, approximately, 30,000 empty spaces at the three grounds combined in 1986.
So the fact that bigger grounds would have made an average crowd of 50,000 is a FACT, is it?
On the point about Internationals being an event, I genuinely believe we could arrange an Ashes series for next month and get 40k at Wembley, 40k at old Trafford and 30k+ at Leeds. I don't think we have seen a massive decline (some impact due to the horrendous performances in the late 90's as I posted earlier), more that the events just haven't been created.
One of the major things I loved about Test RL all those years ago was the chance to go to different grounds. Playing the highest Test matches (against Australia) at regular SL grounds (very good ones mind) just doesn't massively excite me.
It's no coincidence that even disappointing attendances at Wembley have been a lot higher than most heartland Test matches.