I’m not sure they wouldn’t have been used. Those with collectors will still be there. The rest could have been used at any points in the future as those of us buying from Philately can testify. We will never know I guess.
That said, I am not a maths expert or an analyst so if you are a whizz with numbers and I am way off the mark, please let me know
Not off the mark at all 
To add to this because I’m curious and off work:
You looked at first class stamps, but the price ratio between first and second class changes a lot, and it so happens that it was very low in 1971 and was very high in 2022. Its still an increase of 8.25 times, which is huge.
All decades have an increase in cost compared to wages, but the biggest is in the 1970s so I don’t think its because of people no longer sending letters.
This part is based on inflation, because its easier to get that data than annual real wages.
This is a two year moving average because they often have a real terms increase, followed by a real terms decrease, which cancel each other out (it also reduces the spikes in 1975 and 2012 which were about 30%).
Most of the increase happened in the mid 70s and from 2006 to 2012, then 2021/2022. I’m not convinced that its because fewer people are sending letters having seen when it goes up and down.
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