It's hard to predict without SMART features to measure the current wear state. Write amplification from the file system, and from the drive itself if you're not using large block writes, means you can't just calculate - you have to measure.
You can roughly predict the longest possible lifespan for a SSD under a given workload. Regardless, a significant percentage of drives will still die earlier than that.
If you're talking about most incredibly naive SSD storage systems available today (excluding violin memory and maybe xtreme/pure), then I agree with you.
Reads, program/erases, controller ecc/ read disturb management, the g/p list mapping of the blocks... This all has to be taken into account in a dynamic way. And yes, some people are doing this at a higher level than the SSD controller.