This honestly feels unrealistic. Even with someone pulling strings to arrive at this result, professionals would not just trend towards worse and worse results as time passes. They would experiment if it isn't too costly (which clearly the herbs aren't), and they would particularly be trying to figure out the issue of taste (even if a potion is only fractionally as effective, if it tastes good or even fine, you could use it for more minor stuff or get animals and kids to actually ingest it! so there is a market).
No, things would progress. At times it would stagnate or even take steps back, but for the most part things would go forward over the generations, not a steady trend backwards.