Just to add to the discussion - by introducing the 2 x 2 switch stack you are added a level of complexity to the design that doesn't really need to be there. Is the business a 24 x 7 operation 365 days of the year so can't afford any downtime to upgrade the cores? As already highlighted, you also don't gain anything by having the dual stacks as only one will pass traffic to the MX due to STP. I believe in keeping it simple unless the business requirements stipulate otherwise.
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