You (the OP) may be ok in warmish, dry weather, but you could easily find out the hard way on a cold, wet and/or snowy/icy day or especially at night when the difference in performance would be greater between the sets/types of tyre.
There's a reason why F1 cars get black flagged if they fit tyres (even brand new ones) of a different type on them at the same time.
Not ever worth the risk other than for an emergency spare when you drive accordingly - and for a much shorter distance than usual - before replacing it with a proper matching new tyre.
You probably were fine because the vast majority of the driving was in decent conditions and not having to use anywhere near the full potential of the tyres' performance, e.g. in very bad weather.
You want your car's tyres to be predictable and controllable in all conditions for the way / speed you drive at.
The same rule of not mixing and matching summer and all-season tyres also applies to doing the same for asymetric and directional tyres. They behave differently in the same conditions - again, more so in poor weather. It isn't just the case that as you go from summer to all season to winter tyres, they just get softer and wear out quicker.
What you should be doing is rotating your existing tyres enough so that all of them wear out at the same time, and replacing all four with a new set of X, Y or Z. You should then be a bit more cautious as the tyres get scrubbed and you get used to driving with them, compared to the previous set.
Sometimes, the driving style needs to be adapted - especially in poor weather - to the set of tyres you have. Even though my current (2yo) set of CC+s are excellent, they still would not be as good performing as a new summer tyre of the same grade in non-winter (above 7degC) conditions. In winter, they (by their nature) do perform better, relative to summer tyres.
I would say that under the right conditions (that happen more than you think), having a set of new all-seasons and older (worn) summer tyres are just as bad as having a set of new and old summer tyres fitted to the wrong axles.
Edited by Engineer Andy on 30/07/2020 at 13:22
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