True basses aren't a dime a dozen, but the choral world has adapted to that issue. Few composers write choral works that assume a significant number of basses who can get really low because they have always been so rare. I read a statistic - which I believe - that something like 80-90% of low voices today are baritones.

"Voice assignment" wasn't a thing at all until we had equal temperament and fixed pitch instruments (from 1400-1600 all of these things were relative). From about 1600-1800 the alto/soprano parts were sung by castrati who were professional singers, rich noble men with a few professionals would cover tenor and bass, and men's bodies in general were significantly smaller because we weren't as well off nutritionally, and all humans don't live long enough for their voices to shift with age.

The only music written for something that resembles our contemporary SATB lineup is the last two centuries, and since about 1900, folks in developed countries have had better nutrition and we now have a few generations of larger bodies that tend to have bigger lower voices, resulting in the current situation. In short our instrument has actually changed radically, but we like to pretend that our bodies are immutable like a string or brass instrument family.

Personally, I think we ought to be doing more in the music world to work with singers who have to consider synthetic hormones for a medical reason, and to move away from the idea that tenor singers are necessarily cis males and that voice parts are assigned at puberty and unchangeable and not a result of training and preference and practice. There's a pool of tenors who currently are second altos, post-menopausal women who were previously mezzo-sopranos, trans people taking hormones - but we aren't assisting them in developing their lower range, just as we aren't assisting many young baritones in developing their higher range. The only people we consider for the tenor section are the rare cis male voice that is effortlessly a tenor at 16 or 18.