It would seem that the subsidy for the first 4 years (210,000 blocks) could have been 40, 100, or any other value. Was there a reason Satoshi choose 50? Or was that an arbitrary decision?
While I don't have a definite answer to this question, having not seen anything written by Satoshi justifying this choice, an interesting property of starting the block reward at 50 is that, at the end of the 4-year halving period that generates X bitcoin per block, X% of the supply remains unmined. Since the current (2022) reward is 6.25 BTC per block, it's easy to know that 93.75% of the 21m bitcoins will have been mined at the next halving.
Another reason for the choice of 50 may be that a different choice would have changed the 21 million total supply (unless other changes were made as well). It may seem that this total is arbitrary too, but this answer explains why this total works well with 64-bit floating-point representation.