Process for invalidating a patent
Article III sets forth that for matters dealing with property and private rights you need a judiciary that is independent and accountable to no-one except the Rule of Law.
To date, IPRs have been considered by many as being very useful and effective.
For example, these continuing applications may be used to secure patent protection for previously unexamined subject matter (i.e., non-elected inventions) or seek broader patent protection for what has already been granted.
However, to take advantage of the practice of filing continuing patent applications, a to the earlier filed patent application must be followed.
The PTAB (being a creation of Congress) is very much part of The Executive Branch of government.
PTAB judges are selected by the Director of the USPTO; a political appointee.