The yield change is only relevant for those who buy at the revised price. If you buy something for €80 which pays €4 per annum you are receiving 5% yield. If that asset changes price to €100, you are still receiving €4 on your initial €80 - you are still receiving 5% yield. If I buy the asset at €100, I only get 4% but that doesn't change your return until you change your holding.That seems wrong to me. Just arithmetically.
If you bought Stock A at 80 for a 5% yield and now the price had moved to 100 (so now yielding 4% on the market price), you could sell Stock A and buy any Stock B with a yield of say 4.5% (or any number greater than 4%) and be better off that you were.
If you then look around for a better % return, you need to beat 5%. In absolute terms you are thinking you need to beat €4 but in fact that's because you are converting capital gain (the 80 to 100) - you sell the asset and have a larger capital base. But to get the same % yield you need to beat €5 not €4. Which may be a valid choice if looking at absolute € income rather than % yield.