Not necessarily, they can use less dense NAND chips for that & the performance will be virtually the same.64GB is going to be slower than 128GB. Also, limited writes. 64GB has 1/2 the lifespan of 128GB in terms of writes. Not to mention at 64GB, a lot of users would probably find it annoying that the remaining ~40GB available (after system+other) would be further decreased due to swap.
The free space is more critical than the total space, 64GB storage with 40 GB free will wear out the NAND at a slower rate than another device with 128GB storage but only say 20GB free space on it.