Things aren't always that simple. For example AMD may be cheaper per core, but if you're running AVX heavy code then intel has a throughput advantage. At least it did with Zen, Zen2 has some improvements in that area, so we'll have to measure again.

Moving to the next intel generation is usually simple, incremental change. Moving between vendors means a lot of re-evaluating if you're crunching numbers.

If your workload needs a lot of PCIe lanes for IO or you need cheap systems with ECC ram then the story changes quite a bit.