Do you agree with that formula?

If disk failure is relevant, RAID-0 is the wrong answer.

If you're asking from academic interest, you'd need to consider whether striping across disks increases or decreases the likelihood of failure due to different read/write behaviour. (I don't know what the contributing factors to a disk failing are - i.e. is it purely random, actively increased by use, etc.) That formula doesn't include any such parameters; it may well be an approximation of something that does. I don't see on the linked Wikipedia page where the poster apparently sourced it from, so it's currently just a way to produce a number slightly less than directly multiplying the failure rate by number of drives.

Disks can fail in several ways: bearings, actuators, surfàce, PCB. The failure rate increased dramatically after the flooding in Thailand destroyed many mfg facilities, leading to speculation that flood-damaged parts continued to be used. Outside of production servers, the main factors influencing failure rates appears to be make, model and even batch. Some guy running a cloud service periodically publishes failure stats on assorted makes and models.

Indeed they do

https://www.backblaze.com/blog/hard-drive-reliability-q3-2015/

Another important question these days is the capabilities of the drives and the controller (most controllers do share the bandwidth to some degree between the SATA ports), and why using solid state drives isn't an option. For example, striping mechanical hard drives will almost always still provide poorer performance than a single SSD. Especially seeing as modern SSDs can easily saturate a 6Gb SATA link. For ultimate performance, you'd connect a fast SSD to either a SATA Express port, or a bespoke PCIe controller.

I just can't help but think that RAID-0 is a lot more redundant these days than it used to be. Faster performance, increased risk of data loss, 2 drives required, or just slap a fast SSD in instead. Unless of course you need lots and lots of storage, and even then you can now get 1TB SSDs for less than £60...