"So the only danger of not having ECC is exceedingly rare memory errors. If those rare instances mattered that much then you should get ECC but for everyone else how is it worth it?"

The fact DDR5 has auto-error correction for the data that has arrived to the memory chip should tell the story. Still not full ECC as the latter requires cooperation w/ the memory controller, yet the price in material is there already. But yes, w/o any shadow of a doubt - the 1st and most important part of any program is correctness, optimization come afterwards.

The internal ECC of DDR5 is not good enough. Its only purpose is to restore the reliability of DDR5 to the level of DDR4, despite having smaller cells and higher throughput.

The only useful ECC for the user is the one computed in the memory controller inside the CPU, stored in the DRAM and verified after returning to the memory controller.

This allows the CPU to be aware of any error and it also corrects or detects the errors caused by electrical noise on the PCB traces or by bad memory sockets, not only those caused by bit flips inside the memory cells.