Hi, this is a great article. I'll be sharing with my customers.
Just a minor question on the 3-month loss. It is already accounted for in the 4-year maintenance. Also, if the actual migration is 3 months, then the new array should be purchased just 4 months earlier, not 5 months earlier. IMHO it does not take 2 months from racking to Operationalised.
Using the same calculation, the cost of Server SAN (e.g. VSAN) will also be higher due to this migration. But the extra cost will be lower as we can migrate 1 cluster at a time.