A few years back my winter tyres (Nokian Hakkis) got to 7 years old, and were still acceptable but nowhere near as good as new. I thought they would get to April ok, but would need to be changed before next winter. I figured they were no good in Winter, so I thought I would run them over Spring/Summer until the tread wore away and then put my summers on. Despite all the "common wisdom" saying winter tyres wear out rapidly in high temperatures, I ran those suckers all spring, summer and fall and they were still going strong when I changed them with new ones in November.
So much for winter tyres wearing out when it gets hot! They did fine even when I was driving at 90+°F.