It is not up to a beis din to be able to foretell the future or read minds. A person can in no way read minds, and if they purport to, they are not espousing Judaism.
It is up to beis din to witness the conversion and sign the teudah. Not to ensure that a person is observant. That is the ger’s responsibility. Once the ger leaves the mikvah, they’re Jewish.
And, we do not allow Reform and Conservative Jews to dictate halakha. According to your reasoning, that we don’t do something only because they do it, is silly and dangerous at once.
The Reform use ta’amei hamikra. We shouldn’t because they do? Come on now.