You could have an 8520U get clapped by a 7540U for example, because even though the former is newer, the latter is Zen 4 to its Zen 2.

Absolutely completely irrelevant!

Show me a laptop line with these both in them. They are for completely different type of products.

You buy laptops, not laptop cpus. If you are looking for something to game on, you will not even see laptops with mendocino on them.

They had to change the system as they now have multiple different types of cpus instead of one they had different numbers of cores for.