When I look at the cell phone reviews and the costs I don't see how the cost of ownership factors in how long a vendor supplies security updates for it phones. If a vendor only supplies 2 years of updates - for example Motorola (after first product ship) and another vendor provides 3 years - e.g, Google Pixel and Apple provides 4-5+ years, isn't that an important factor in considering the useful life of a mobile phone investment? It seems that the cost of ownership per year would be pretty different after factoring in this "feature."
Am I putting too much emphasis of getting the monthly OS security updates?
Yes how long a phone may get updates should be a factor in picking a phone, but I would put it down as a very minor one. Most issues with updates on phones from an end-user point is not as much to do with security but with new features or having the right version to run a program. Most people use phones that are several years out of date with the current OS version without any issues. It's also not very easy to find out how long a phone maker would guarantee updates on a phone, and I doubt it's something many people ask about.