I respect the decision of every Monarch to abdicate or not to abdicate, but in my opinion an abdication has mostly if not only advantages for every party directly or indirectly involved:
1. The King / Queen who abdicates can proudly watch his/her daughter or son succeed and carry on the tradition. And can enjoy his / her last years without the burden and responsibility to be King / Queen.
2. The new King or Queen can come to terms with his / her new role and changing life without having to mourn his / her father or mother.
3. The countrymen of the new King or Queen can celebrate their new sovereign(s) without the touch of sadness that comes with the dead of a (former) head of state.
I am sympathetic to the idea that a head of state should not hold on to his or her office at an advanced age, let's say, into his/her 80s or 90s. On the other hand, I think that there is a risk that the act of abdicating, which was once confined only to monarchs who were
forced to step down from the throne, might now become trivialized.
In fact,
voluntary abdications, once restricted mostly to the Benelux countries, now seem to be slowly becoming the "new normal", see recent examples in Japan, Denmark and Spain, besides Belgium and the Netherlands (although it is debatable to what extent King Juan Carlos I's abdication is Spain was voluntary or not).
In most of those cases, however, I suppose that there
could be an alternative to abdication, which would serve the purpose of replacing an aging head of state while at the same time preserving the life-long nature of the position of monarch, which distinguishes it from that of a president. I am referring specifically to the option of setting up a permanent regency under which the monarch gives up the exercise of the role of head of state while remaining nominally the king or eqivalent, i.e., no succession to the Crown takes place.
The obvious difficulty is that regency laws, as they are currently in force in many countries, do
not provide for the possibility of a regency being normally used as a proxy for a monarch's "retirement". On the contrary, the law normally only allows for a regency to be established when an adult monarch is patently incapacitated, either physically, mentally, or both, as in being literally unable to discharge the duties of his/her office.
Of course there is some room for flexibility, depending on how the law (or the constitution) is interpreted. The challenge is, however, that, if you make regency rules too broad or too flexible, there is a risk that an able and young king may be vulnerable to being removed from his office at the discretion of the government or Parliament, i.e., the monarch doesn't have any real constitutional guarantees. Conversely, if regency rules are too narrow and limited, the possiblity that I suggested of using a regency as an alternative to abdication for an aging king or queen is severely restricted unless, as I said, the monarch is truly incapacitated.
Personally I think that being a regent for an aging father or mother is a good experience for a crown prince or crown princess, as it gives them practical training in the role of head of state without the burden of assuming the
full dignity of a monarch. I suspect that many of the kings who ascended the throne in their 40s or early 50s in Europe recently, as a result of abdications, would have been satisfied with, or might have preferred the option of being a permanent regent instead, if they had that option (as Hereditary Prince Alois in Liechtenstein for example).