Bcrypt does not add entropy, it only adds "difficulty" and the problem with "difficulty" is that it breaks down over time. Improvements in technology and processes routinely undermine the difficulty estimates and corresponding factors used to tune KDFs like bcrypt.
KDFs are good at protecting "okay but not great" passwords used to gain online access, but they add no protection to extremely secure, unique passwords, and they don't add enough protection to extremely weak, common, or reused passwords. They are there to frustrate attacks, not make them physically impossible. Many credentials are time-sensitive, and many attacks are not targeted; KDFs are good in these common situations. You still need to pick a password that will take long enough to crack that an attacker moves on instead.
However, some data needs to be protected practically forever, and some attacks are definitely targeted at specific people or systems. In these cases, KDFs don't do very much. Taking a 256-bit key just from the raw bits of 32 random ASCII letters and numbers will already get you 190 bits of entropy and frustrate all practical attacks for the next several decades at the very least. Feeding that through a KDF first won't add any practical security. Even so, KDFs can be used for a different reason, enabling passphrases, which are long strings with low per-character entropy but high overall entropy. At least, assuming that the KDF preserves that entropy well.
Regardless of bcrypt, you should always pick security keys with sufficient entropy, where "sufficient" is measured relative to the importance of the thing being protected, how long it remains important and accessible by that key, and what attacks are viable now and foreseeable in that time.
KDFs are good at protecting "okay but not great" passwords used to gain online access, but they add no protection to extremely secure, unique passwords, and they don't add enough protection to extremely weak, common, or reused passwords. They are there to frustrate attacks, not make them physically impossible. Many credentials are time-sensitive, and many attacks are not targeted; KDFs are good in these common situations. You still need to pick a password that will take long enough to crack that an attacker moves on instead.
However, some data needs to be protected practically forever, and some attacks are definitely targeted at specific people or systems. In these cases, KDFs don't do very much. Taking a 256-bit key just from the raw bits of 32 random ASCII letters and numbers will already get you 190 bits of entropy and frustrate all practical attacks for the next several decades at the very least. Feeding that through a KDF first won't add any practical security. Even so, KDFs can be used for a different reason, enabling passphrases, which are long strings with low per-character entropy but high overall entropy. At least, assuming that the KDF preserves that entropy well.
Regardless of bcrypt, you should always pick security keys with sufficient entropy, where "sufficient" is measured relative to the importance of the thing being protected, how long it remains important and accessible by that key, and what attacks are viable now and foreseeable in that time.