The whole public policy justification for patents is to bust trade secrets into the open and reproduce the knowledge in the public domain, and to facilitate private work on research and development that otherwise would not occur due to the inability to preserve exclusive knowledge of the results.
In exchange, the beneficiary of the patent is granted a time-limited trading right, is relieved from any costly measures he might have needed to take to protect the secret, and is facilitated to develop a trade which might not even be feasible to start without the initial protection of the patent.
Once the patent expires, the assumption is that the patent holder should already have covered his R&D and startup costs, established himself in trade, and therefore continues to have an advantage against any new competitor.
If someone else takes the documented patent and immediately uses the insights within it to develop another thing, then that continues to contribute to the public policy. That is so even if the original patent is completely superseded and effectively rendered worthless.
Obviously, the patent holder is deprived of the economic benefit he expected it his patent is superseded, but then it's up to him to stay at the forefront or else fall by the wayside.
Also, it's not always possible to develop a system that copes equally well with all circumstances, especially hypothetical ones. In some cases, we have to accept systems that cope well with typical circumstances or on average.
The public policy purpose of patents is not to guarantee the enrichment of holders or to reward them for work actually done, it is to give holders a particular chance at enrichment in exchange for publication of their knowledge. It is the person with valuable knowledge who can decide whether he wants to play that lottery or not.