Might I bring forward, as addition to all that has been said here, a viewpoint of Bikkhu Analayo, who wrote (p. 170) that the degradation in the Dhamma we perceive might not just be because bikkhunis were ordained once but because they are not so now?
https://www.buddhismuskunde.uni-hamburg ... kkhuni.pdf
... and also by Bikkhu Analayo, a discussion of the Four Assemblies as per the Mahāparinibbāna Sutta, here: https://www.buddhistinquiry.org/article ... -buddhism/ (the Sutta itself, here: https://www.accesstoinsight.org/tipitak ... .vaji.html)."When considered from the canonical viewpoint, however, the decline of the sāsana is rather to be found in the absence of an order of bhikkhunīs. A discourse in the Aṅguttara-nikāya and its Chinese and Tibetan parallels describe several unfortunate conditions where one might be reborn and be unable to practice the Dharma, such as being born in hell, in the animal realm, among hungry ghosts, or in a border country. The problem of being born in a border country is that there the four assemblies are not found — the bhikkhus, the bhikkhunīs, the male lay followers and the female lay followers.
While this passage speaks of the absence of all four assemblies, in view of the interrelation between the four assemblies, even the absence of one of them would result in a deplorable situation.
From this viewpoint, Burma at present would be in such a deplorable situation, lacking an assembly of bhikkhunīs in the country. In Thailand the prohibition of bhikkhunī ordination by Thai bhikkhus makes it difficult for Thailand to emerge from a similar condition. Only in Sri Lanka has the border country condition been fully overcome, as all four assemblies can now be found in the country, although official recognition of the bhikkhunī order by the Buddha Sāsana ministry has so far been withheld. The border country condition has also to some degree been overcome in those countries in the West where, even though Buddhists are a minority, bhikkhunī communities have come into being."